User blog comment:Martialmaniac/Illumi's necromancy/@comment-4828844-20151021095057/@comment-25814498-20151021184245

Actually, it could just be that they were only alive as Needlemen during the Skype call with the entire Mafia community just before the auction(as that was also part of Illumi's contract), and then after their job was done Illumi dispelled his Nen and they all died from all the Needles stuck to their brains. But because they were so well hidden and only a few members of the Mafia knew their location(namely, everyone present at the mansion and the Shadow Beasts), nobody found out that they were dead and so there was a delayed reaction to their deaths.

As for why he clicked his magic beeper before the deed was done, it could be just as you said. Illumi wanted to get payed for his contract and knowing that there was no chance of the 10 Dons surviving, pressed it prematurely.