Does Seam mean he would return to Ireland and fight the British, instead on leading the French resistance?
He doesn't have that much influence outside of Paris.
Why would Santos sell Sean out to the Nazi's if Sean's his best customer?
He was pushing the Nazis to a point that that was bad for business? Even one man can only give so much revenue compared to supply both sides in a stalemate. Or he was approached and made an offer he couldn't refuse.
A decisive victory for either side would have been bad for business.
While we're on the subject how Santos was even able to sell out the Resistance to the Nazis?
Sean is his only customer and he hasn't said a word about HQ's whereabouts, so how in the hell Santos knew where they were? The Belle I can still understand because he operates next to it, but the rest crosses my Willing Suspension of Disbelief because there's no logical sense how he also knew about Slaughterhouse and Catacombs.
He talks about Sean's presence in the Belle early in the game and he is present in the Slaughterhouse after one mission, so those two he does know about. As for the catacombs, the black guy says it is the one place the Nazis dare not go, which could mean that they know about it but are leaving it alone.
How come Nina Simone's "Feeling Good" is included in the soundtrack, even though the song wasn't written until 1965?
For the same reason "Koop Island Blues" was, and that wasn't written until 2006. The songs for the soundtrack were selected for atmosphere and mood rather than historical accuracy.
Is Vittore Sean's dad?
As he dies, he calls Sean "son". Now was he being literal, or was he calling him that because he considered him as such? Is he the father we hear a few time having had gotten in trouble and kicked out of Ireland?
Probably the latter, since Sean isn't Italian in any way and he doesn't refer to Vittore with the vitriol you would expect if he's the reason Sean got kicked out of Ireland.