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.
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.