You don't actually even know this man was your father, just your mums word that it was only these two. You say he didn't know you existed but you also claim that both decided not to be dad's and left, so which is it?
Even if he knew about you, 50% chance of parentage would not have me committing financially to being a father, I'd want proof. Your mother obviously didn't seek to provide it and seemingly didn't really need to as other man paid instead.
I don't think you have any right to his estate whatsoever to be honest, and I think it's a pretty vile way of looking at things. I'm sorry this has been your life but I don't think you should be marching in to upset the family of a man you've stated didn't even know you existed.
If he were alive, then fair enough, contact him. But he isn't. It's over.
Re the maintenance that was paid, no I do not believe he can claim that back. He accepted parental responsibility without proof, foolish but not something he can now claim for.
Again though, you say the dead father was rich whilst you and your mum suffered, but you WERE being supported elsewhere so this isn't entirely accurate.
Sorry OP, but I don't think you should be acting on this financially.