Are all atheists this ill-informed about religion?

[Pat Mustard]

Is a baptism considered legitimate if it's performed on an infant? 

It is for catholics, even if the infant's parents don't want the child baptised.

Does one even have to be baptized to get to Heaven or does one gain entrance through grace alone?

It depends on how much of a Fr. Trendy the clerical "expert" you are currently talking to is. In Ireland up until very recently, unbaptised dead babies weren't allowed burial in cemetaries, because catholicism.

Is simply accepting the Holy Spirit considered baptism? Or is actual immersion required?

Catholicism demands some form of either dipping the head in water or pouring it on.

Strictly speaking, baptism in water isn't strictly necessary according to scripture, but because John the baptist is such a big part of scripture, and Jesus was supposedly baptised by John to fulfill some prophecy made up after the fact, most of the major christian denominations have some form of baptism. Actually here's a nice little chart of differences between denominations.