He had just demonstrated that a) he'd lied to a terrorist wing of the IRA and b) he's kind of an idiot. Even if Dierdre was willing to leave him alive (unlikely), Seamus certainly wouldn't be. Dierdre made a show of bribing/threatening him into silence because, if she'd just executed him right there, the rest of the team might think they'd turn into a cost-cutting exercise after the heist and the whole plan would fall apart.
Honestly, I was half-expecting him to show up during the climax, maybe working for the Russians. Still, I think that if that was supposed to be the idea, the film would have shown or at least hinted at it.
"Other old man" meaning the old man sitting in a chair in the photograph who isn't the boss called Old Man. In the pre-mission blurb when she goes to assassinate the General, the Ronin mentions that he and she were taught by the same mentor. The old man's clothing resembles a traditional Samurai, and his absence in the game could be due to the Mentor Occupational Hazard.
No idea who the sickly-looking man is.
- Isn't it obvious? It's Mr. Poe from A Series of Unfortunate Events! Guy has terrible luck with orphans...