This borders on a dumb question, but do the 10 nights from the Ibis card and the 20 nights from the Voyageur card stack? I ask because that would equal "instant Gold". I'd presume it doesn't, but...well, let's face it, this is FlyerTalk we're talking about, so I also think it would be silly not to ask.

(Annoyingly, the only Accor brand I have much use for is Fairmont courtesy of its old railroad hotel heritage...)

They stack.

And I think the Accor Plus do as well. Not sure.

The Brazil Signature status points do as well, but if you are chasing status points, you will not be interested in nights !

I have both Ibis, Voyageur and Signature. I did bought the Ibis just as a fast track to platinum, well worth it, 10 additional days with lounge access and free breakfast in Asia.

Accor Plus will not stack. You don't get another 20 nights if you already have Voyageur.

Accor Plus can't stack with Voyageur or Ibis card, but Accor China ALL Plus program can stack with Voyageur and/or Ibis card

So if I understand correctly, Voyager (20 nights) + Ibis (10 nights) +Accor China ALL Plus (30 nights at the top) could kick out 60 nights (i.e. Platinum)? Yes, that'd be $600, but given that Fairmont is a strong preference for me while in a number of places in Canada (and Diamond is a dog that just ain't gonna hunt)...

Correct, but it really depends on what you are actually looking for. The core Platinum benefits are lounge access and free breakfast in APAC. None of which applies to the places you are planning to stay (benefit does not include Fairmont Gold Lounge and breakfast is APAC only).
For Diamond status, these cards are of no use as Diamond count on status points only which can only be boosted by ALL Signature subscription.

Yeah, I know. Like I said, Diamond with Accor is a dog that won't hunt.

Early check-in/late check-out has a benefit. The extra points earnings are non-trivial if not massive (presuming a spend of EUR2000, that would be EUR76 in bonus points). Premium internet is also not a trivial consideration (I'd mark this down as being around $100 or so in benefits based on the DC hotel's relative rates). I know there's some inter-currency scratch-pad math, but that would come to about $200 in directly benefits (give or take) before various upgrades and so on (I presume I'd hit three suite upgrades per year - two from status, plus about one from spend) come into play.

[Honestly, ALL is possibly the weakest program out there of the major hotels. It's only the fact that I really like the old railroad hotels in Canada (they're one of very few places that I can reliably get a decent steak tartare) that has me sticking with them. When it was "just" Fairmont things were rather easier/better IMO.]

Based on the stay pattern you described, you will only get 2 Suite Night Upgrades. You would get 2 SNUs upon reaching Platinum via your 60 nights. However, the 3rd SNU only comes at 18,000 status points. If you were planning to spend enough to earn 18,000 status points, then you would naturally qualify for Platinum, and would have no need to pay for any of the additional programs you're considering to reach that bar.

Also, if one earn 18 000 it is an obvious choice to sign up for Signature, which gives another 12 000, thus well over the threshold for Diamond and many more SNU.