Cullen is Alistair's long lost brother of course. 
Why? Cullen never said he didn't know who he father was.
Fiona did have Marc's baby and she did have Duncan take the baby to the King because a female Warden would have to leave the order if they had a baby. So where is the baby of the man who had three relationships. Katrina died no child. Rowan is Cailan's mom.
Is it your belief they dump the kid in the trash? That doesn't sound like Marc. So where is the child that is about Alistair's age. Was Marc involved with the maid? I wonder why David didn't write him as a womanizer.
If you have a conversation with AL, he states that Duncan had a relationship with him before becoming a Warden. He states that Duncan knew he wasn't happy and he argued with the Grand Cleric to get him out.