I feel like there was a lot of different views when it came to voting the tiers. Personally I voted what I thought "wouldn't tear this tier apart" but something like "I don't want people who play lower tiers to get overwhelmed by this particular mon on release" popped in my head at times (Which would probably explain some of the conservative decisions you mention, but that's just me, I'm one out of 9 people), others seemed to aim for more of a "I predict stuff will look more or less like this in the end" others, I don't know.
Hidden abilities won't be out on release as far as I know. They will be released on the upcoming dungeons as addressed by Kyu I think? in a post on some other thread, so overall this entire thing will be worked on the assumption that HAs won't be out, and the changes on mechanics only we currently know.
Yes, gems will be implemented.
Agreed. I believe it's going to be an amazing anti-meta Pokemon as Mence's one true counter, and a good way to deal with a lot of other popular picks, the only way I can think it'd be UU by usage is if people opt to use their one Eviolite (Until they replay the game on an alt, buy it off someone else, or dunno if it will be purchaseable with BP) on Chansey the majority of the time, however it is a case where I thought "This Pokemon won't tear the tier apart" so I voted for it to be allowed in UU.
Like I mentioned earlier a case where I thought "I don't want all the NU players to suddenly be dropped this 160 attack beast without any time to prepare". Some guy may want to try new stuff but has to suddenly deal with one of last gen's hardest hitters.
EDIT: LMAO I missed out on most of the post while I made this will edit shortly
EDIT EDIT: Nevermind it'd take a decade to respond to all of them. A bunch of them I agree with, others can be summed as "this has no counters in lower tiers", others are you just saying "lmao" cba to go into specifics.