Salam, Sister-to-be Adriana.
May Allah guide you, bless you and reward you for your choice of Islam!
Let me respond to your questions one by one.
First, I will answer your question about a Muslim leaving Islam.
Actually, there is a lot of confusion about this issue of apostasy. As you have noted, the Noble Qur'an does not prescribe death penalty for deserters of Islam, but rather states that they would be in Hell in the hereafter (2:217).
Here is a verse that directly refers to people who abandon their faith after accepting it. It says what means:
*{But those who reject faith after they accepted it, and then go on adding to their defiance of faith never will their repentance be accepted; for they are those who have [of set purpose] gone astray.}* (Aal-`Imran 3:90)
As you can see, there is no mention here of the death penalty for the deserters.
But there are Muslim scholars of respected opinions who believe in the death penalty for apostates because of hadiths that mention it. If we study the context of these hadiths, we can see that the ruling was with reference to certain specific cases of miscreants who wished to undermine Islam, by joining Islam first and then deserting it. This is mentioned in the following verse that says what means:
*{A section of the People of the Book [Jews and Christians] say: "Believe in the morning what is revealed to the believers [Muslims], but reject it at the end of the day; perchance they may [themselves] turn back [from Islam]."}* (Aal `Imran 3:72)
In order to protect Islam from such malicious attempts to subvert it, the Prophet ordered the death penalty for such people. This indeed was a drastic step adopted by the Prophet to ensure the solidarity of the nascent Muslim community in those days. If it had been a general ruling applicable for all occasions, this would have been stated in the Qur'an because it is a life-and-death question that affects the very fabric of Islam most seriously. On the other hand, read the verse that says what means:
*{Again and again will those who disbelieve, wish that they had bowed [to God's will] in Islam. Leave them alone to enjoy [the good things of this life] and to please themselves: let [false] hope amuse them: soon will knowledge [undeceive them].}* (Al-Hijr 15:2-3)
This verse clearly indicates that the disbelievers should be left alone. Moreover, the killing of apostates would undermine the freedom of will Allah has bestowed on each human, as is made clear in the verses that say what means:
*{If it had been thy Lord's will, they would all have believed, all who are on earth! Wilt thou then compel mankind, against their will, to believe! No soul can believe, except by the will of God, and He will place doubt (or obscurity) on those who will not understand.}* (Yunus 10:99-100)
*{Say, "The truth is from your Lord": Let him who will believe, and let him who will, reject [it]: for the wrong doers We have prepared a Fire whose (smoke and flames), like the walls and roof of a tent, will hem them in: if they implore relief they will be granted water like melted brass, that will scald their faces, how dreadful the drink! How uncomfortable a dwelling (resting place)!"}* (Al-Kahf 18:29)
*{Let there be no compulsion in religion: truth stands out clear from error: whoever rejects evil and believes in Allah hath grasped the most trustworthy handhold that never breaks. And Allah heareth and knoweth all things.}* (Al-Baqarah 2:256)
The verses quoted above are categorical that complete freedom of will is a condition for sincere and wholehearted submission to Allah Almighty, and no force should be used in this regard. This is because coercion does not go along with freedom of choice.
In addition, we may consider another verse, too, which says what means:
*{Those who believe, then reject faith, then believe [again] and [again] reject faith, and go on increasing in unbelief, — Allah will not forgive them nor guide them on the way.}* (An-Nisaa' 4:137)
The example of a person who vacillates between Islam and disbelief is cited here. Note the expression: *{believe, then reject faith, then believe [again] and [again] reject faith}*.
How can we imagine a person becoming a Muslim a second time, after rejecting faith once, IF he is to be killed after his rejection? The significance of this verse is one of free choice; though one is expected to be responsible in the use of that choice.
Thus we see that the general teaching of the Qur'an is one of freedom of choice and mercy. And if we are to adopt the death penalty for apostates today, the first problem is the absence of a true Islamic system (even in the so-called Islamic countries) to judge apostasy and to enforce the punishment where it is due. Second, there are scholars of respected opinions who doubt whether death is the due punishment now. Third, we need to question the emphasis on punishments rather than on peaceful and diplomatic means of promoting the Islamic ideal. Which is prior to what, in which case, should be judged only by true correct and wise Muslim scholars. Please check the links at the end of my answer, where you will find the Shari`ah's stand on apostacy.
Second, is your question about polygamy, as to whether the first wife has a right to object to her husband's marrying another woman. According to scholars, the Shari`ah does not require the husband to get the consent of the first wife for a second marriage. If the conditions for a second marriage are right according to the Shari`ah, he can legally go ahead and marry. But we need to understand the spirit of the Islamic ruling first.
For instance, if the second marriage is likely to upset the first marriage and the family structure already established, then a second marriage should be avoided, as it undermines the very purpose of the Islamic rules of marriage. But the permission of plural marriage is given in the context of protecting widows, as they need to be taken care of. In the very same verse it is stated clearly that if a man cannot treat his wives with equity, he should not marry more than one woman. This is a clause usually ignored or taken lightly by many men, depriving women the rights Allah Almighty has granted.
It is also worth mentioning that the bride has the right before marriage to demand her fiance to agree in advance to certain conditions of hers, in case he is interested in pursuing another marriage later on. Actually, this can be part of the marriage contract. As it is the duty of Muslims to fulfill all obligations, the Muslim husband cannot but honor such a condition. If you need to know more about the first wife's rights concerning the permission for the husband to pursue a second marriage, please refer to Ask the Scholar service in order to get more familiar with the logistic details of the matter.
Third, you ask about adultery and its punishment.
The first point to note here is your mentioning of punishing adulterous women only. No way! The prescribed punishment is for adulterous men and women. As for the details for when and how the punishment is to be applied, this needs a detailed answer from a scholar of Shari`ah. Some scholars of respected opinion discuss the point that there is no Qur'anic verse that clearly states stoning as a punishment. The majority insist on applying this punishment, which was originally prescribed in the Biblical Law enforced on the Jews and was mentioned in the hadiths of Prophet Muhammad. It is, of course, understandable that the punishment prescribed by the Jewish Law could have been implemented by the Prophet in some cases. Yet, you'd rather seek the opinion of our Shari`ah section on this matter. They would definitely giving you the opinion of a specialized scholar in Shari`ah.
Another point you raised is about the miscarring of justice, namely the possibility of inflicting a terrible punishment on the innocent. This is very real and it is relevant to the law-enforcement departments in ANY system. In fact, the Islamic jurists have stipulated certain strict measures to prevent punishing the innocent. But even then, there is this possibility when the authorities are insincere and hypocritical people. Only Allah Almighty can prevent such lapses and punish those who carelessly handle His laws.
We can only pray that truth triumphs and justice is done in every case.
And Allah knows best.
Do not hesitate to write to us if you have more questions.
Thank you and may Allah reward you!
Salam.
Useful Links:
Why Is Apostasy a Sin?
Freedom of Religion
Islam & Freedom of Belief
Freedom of Belief & Minority Rights in Muslim Countries
Apostasy: Definition & Ruling
Polygamy between Islam and the West
Polygamy and Polyandry
About Divorce and Polygamy
The Messenger of Peace... A Man of Polygamy?!
The Concept of Punishment in Islam
Wisdom of Shari`ah
Are Raped Women Punished in Islam?
Why Are Fornicators Tortured?
Punishments for Wrongdoers… Why?
Islamic Criminal Justice: Is It Barbaric?
Inserting Conditions in Marriage Contracts