Spence was killed off-camera immediately after the team leftHe 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 let him leave 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. Possibly, 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 shown or at least hinted at it.
The other old man is the Ronin and General's mentor."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...