1. I've just been listening so some recordings of 3/1/2017, where I don't think Jon sounds much worse than in the Greenville show. Yes, there were some shows were I though "OMG, he's gonna have to cancel the rest of the tour" (not that he'd ever actually do that ), but as someone noted, he and Phil looked not too healthy, so maybe he had a cold or something, which contributed to the problem... Actually I believe he has indeed been improving lately, for example in THINFS, which would add up to the theory. And he has admitted himself that he's "not quite there yet, but getting there"...

2. Richie going to rehab is an entirely different thing than Richie bailing on him and the band, not even calling personally or giving any reason (if Jon is to be believed). If it really was that way and broke his heart the way he said it did, then I can totally understand how that can cause some kind of depression or related suffering.

3. Yes, there's been constant decline since the late nineties, but the emphasis here is on "constant". Even in early 2013, he sounded just fine. A little darker voice, and he couldn't hit all the notes he could 10 years before, but more than fine. After Richie leaving, it went more and more downhill (in the summer it was still ok and then it got way worse).

4. If it were indeed psychological (as Jon himself has at least hinted himself), that would at least explain why (besides the "normal" decline I've covered in 3.) he can still hit out notes where you'd think he wouldn't be able to (in Wanted, LOAP (the backing track part before the solo excluded of course), Blood On Blood, and so on), and then fail on the easy parts like the verses of Bad Medicine or Born To Be My Baby. I can hear there's different techniques being applied by him there, but the question is why? If it's insecurity, it can be fixed, if it's not then he must be fully aware that he can't hit the high notes any other way and just go for it, no matter the cost. And that would make things worse, perspectively...