Oh, I respect that there are a lot of issues with this team, especially on the special side. But looking at your threat evaluation, I'd guess I've at least covered almost all threats that are physically based.
The issue with being easy to wear down would be true if chesnaught wasn't really good at keeping his health really high. Tentacruel struggles like crazy, yes, so I sometimes have to make some plays with Sylveon to get some wishes. It is an absolutely enormous concern if I have to handle Clefable after greninja, but luckily Clefable needs +2 to start to bruise (I think it get 14% at +1 with moonblast). Rhyperior's lack of recovery is always going to be concerning, but his targets in bird spam will not be able to do much, anyways. The fact that I can literally lay on slowbro for 90% of physical mons (or what feels like that, anyways) indirectly saves Chesnaught and Rhyperior [especially chesnaught, since all it has to concern itself with is R-wash (Ohko), Azumarill w/bd and Exca/Sharp (who it gains health on)]. And I absolutely agree with doublade. It's just I can be really stubborn with giving up coverage to let rest be a useful option.
Going to lose to irregardless:
TG manaphy is probably going to shock this team unless I change sylveon to chansey. I've been thinking of that for a while, since I can technically really lie on Tentacruel to take care of Sableye-mega. (Btw, I think I can get past spike stack stall for the exact reason of acid spray... It may be an ugly process, but -2 > +1)At this point in time, Mega Lati forms kinda fall in the same category... that is nasty bulk that I don't have power for. I'm not sure Chansey even begins to solve this problem anyways. I've yet to see Mega Altaria go special, so I'll take your word for this.
Probably could remedy without too much issue:
With mew, I think I can get it with Tenta same method. Seems ridiculously shaky, but changing at doublade for heatran to solve that or rhyperior for Gliscor just gives me more issues than I started with unless Bro can take even more abuse from Mence/TFlame/Pinsir-mega and such. It would help with Zone, though. Thankfully, I can at least kill the Spdef Gliscor, which seems to be more common. If it's that godforsaken sub toxic gliscor, at this point, I'll take my loss if Sylveon can't get it. While I do take toxic damage, I also have heal bell for his protect turns and 4hko when factoring in poison heal and protect.
Still questioning:
The lati+Gardm thing... Are we just assuming constant abuse from the two is going to break doublade? Because individually, neither can do better that 3hko and Gard is OHKO'd, Lati 2hko'd by gyro. Yeah, there's a good bit of pressure on Sylveon to get a heal bell in case of a crit, but doublade should be able to take enough hyper voices to rest again (although that is a rather unappealing answer). Of course, this discludes Wisp and Shadow Ball, though the former is still hurt by gyro for a 2hko.
With roar heatran, I was incredibly tempted to run HP Ground on Sylveon to punish that switch but giving up bell would be too much for that trade (Protect is absolutely necessary given wish and scouting zone's lock right now). I guess solace in the fact that it can't really kill much besides chesnaught and sylveon?
I don't mind the sleep talk change at all, Doublade tends to hit like a wet cloth with shadow sneak anyways. I might miss it on some really screwy scenario where I wish I had priority but that's a failure on my part for letting a situation like that arise. As for Chesnaught... I'll keep monitoring his average health on this team. I just don't know if it falls low enough to justify that over the cute leech seed residuals. Even taking full health azumarill and rotom right after another, he only loses about 50% health (of course, rotom has to be down 12-17% for this to work). The biggest hits he generally takes is from a landorus-t U-turn, which does about 33%. Same deal with a bisharp/exca iron head, but again, drain punch nullifies all damage here.
I could see going to Spdef Gliscor and Chansey as opposed to Rhypeior and Sylveon. If course, this makes me more reliant on Bro, but it gives me more reliable recovery, a mew/gliscor answer, a TG manaphy answer (not an rd one tho) and a decent helper with Lati/Gard and roar tran. The other fix, which still leaves me open to TG manaphy is to make the Spdef Gliscor replacement and then switch in some physically defensive EVs on sylveon, which helps handle lati@s better. Sylveon is nice simply because it provides an early game check to Sableye-mega if I see a reason to hold onto tentacruel, and it also does have the ability to prevent Mence from mindlessly waltzing in.