Heaven

Allah says in the Qur'an that the only thing that will not be forgiven is that partners be associated with Allah, and that Allah forgives whatever else Allah wills.

Allah also says in the Qur'an that whoever believes and works righteous deeds, including Jews, Christians, and Sabeans, will have a good afterlife.

With that in mind, how can someone exclude some people from heaven just because they are not Shi'i (or Shi'i Ithna Ashari)? 

Only Allah has the authority and power to send people to heaven or hell. 

Nowadays, also, people often follow religions or sects due to inherited understandings and doctrines. A Sunni who loves certain individuals (who do not have the same status among Shi'is) does so because they have heard very good things about them, or because they believe the Prophet (S) loves them, not because they are trying to fight against the Imams or support wrong.

It is a different situation for someone who actually was alive during the time of Imam 'Ali (A) and saw him and rejected him or physically fought against him. Anyway most Sunnis, and most or all Sufis, respect Imam 'Ali and other Imams, they just have a different understanding of their role. 

However, following the teachings of Ahl al-Bayt (A) through the Imams can help develop one's ethics, actions, and spirituality in such a way that can, insha'allah, help with going to heaven. Also one can hope for the intercession of the Imams (A) if one has a close relationship to them and is dedicated to them, and this is also an invaluable gift. 

Allah says in the Quran that no one will bear the burden of another. We are all responsible for our deeds.

It is not healthy to go through life with a sense of pre-destination that one will go to hell, because that can easily become a self-fulfilling prophecy. 

Sometimes some people are irresponsible when they talk about illegitimate children and either say things without thinking, or else they don't critically examine the texts they are talking about and quote them without thinking or comparing or examining them closely. It perhaps doesn't occur to them that some of the people sitting there listening are not of legitimate birth. There should be more responsibility in discussing these things so that people do not go around worrying they will go to hell because of what their forebears did. 

It is said that the garden that Adam and Eve were in was a different garden than the one that people to go eternally in the afterlife.

Allameh Tabataba'i brings up this question in Tafsir al-Mizan (available online) in the section on narrations 2:35-39 and discusses it in light of the narrations. For more elaboration on this question, as well as relevant narrations, where the garden might have been, and so forth, you can read that section.

I have not come across the idea that the angel Mika'il keeps the devil out of heaven in mainstream Shi'i or Sunni texts.

Also, the Qur'an says that God Himself threw the Shaytan (the devil) out of heaven. This is not to say that an angel could not have been involved; however, the Qur'an does portray it as being directly between God and Shaytan. It also implies that Shaytan accepted being thrown out insofar as Shaytan swore to misguide human beings until the end of time (and this oath implies that he is not trying to return to heaven).

Of course, God knows best!

A summary of what the angel Mika'il does as per Islamic texts can be found here: https://www.al-islam.org/ask/topics/4478/questions-about-Angel