I love your story ! out for the part what you discovered i have the same history, my granddad on my dad side is raised by his grandparents, he had his mothers name . I was too young to have been curious of his past, he died when i was about ten i think. His mother married an other man and had 3 other kids, but we do not know who they were. My granddad himself was not that steady either, he first married a woman he had 8 kids with, but he said the only one his was the oldest, and the rest was of his cousin, who we also do not know :-) than he married a woman who died leaving him with an other daughter, and than he married for the third time with my grandmother who he got 3 children with. My dad had uncles he never saw or knew about, my granddad never wanted to talk about his past, not about his grownup years nor about his childhood.
That you talk about the difference of DNA ! i never knew that, so there is a possobillity you can do a dna test to find out familynames of possible famuly?