So he lost alot of money, that's motive for getting it back. I can't see the link from 'i want my money back' to 'i'm going to blow you to smithereens'. If the plan had worked out, it wouldn't have mattered real or fake, he'd have his $3M. And it'll take alot less time, less risk of accidental detonation and be easier to implant.

Unless you're saying that he took the loss personally and there is only one part which alludes to it, where he was 'humiliated' by the hotel staff asking him to vacate the room that was reserved for him as a former high roller.

I'm not saying everything's unicorns and fairies but I didn't get the motivation to cause actual harm and therefore the plan felt wasteful to me to have a real bomb. And why would he say that if 'there was a double cross, I'll come back at a later date to do it again'. That just totally ignores what he said at the beginning of the letter about not giving instructions of there was any deviations to the plan and also contradicted to the bomb being real and destined to go off anyway with a unstoppable 3rd timer.