"1. Are older MSS better? They used the Sinaitic & the Vatican Greek texts, 2 of the oldest & most
corrupt MSS ever made, found in trash cans in a catholic monastery. ?"
For the most part, yes. This is standard textual criticism practice, and by no means is it limited to biblical textual criticism. Older is better. But besides the two you've mentioned, there are actually quite a few other manuscripts even older than they are that tend to support modern textual critics against the Textus Receptus used by the KJV.
Only the Sinaiticus was supposedly "found in the trash." But either this was because 1) Tischendorf claimed this so that he could steal it from the monastery, or 2) It was accidently misplaced/ thrown away. Either way, a 1500 year old vellum codex in good condition is not trash, regardless of the contents. It would fetch a high price from antiquity dealers even if the contents were drivel. And, by the way, appealing to the "manuscript in the trash-bin" argument is a textbook example of an informal logical fallacy!
"2. They used these MSS because they agreed with JW's rejecting the Deity of Jesus. ?"
Interestingly, the older manuscripts do support JW's positions better than the later manuscripts. This is consistent with the belief that Christianity gradually got more corrupted over time - a main JW teaching.
But regardless, most newer Bible translations use the same textual basis that the NWT did, including the NIV, NASB, HCSB, NJB, NAB, etc.
"3. The JW's changed the references to His Deity in over 23 places. ?"
We'd have to see the particular places you're referring to. But it is correct that the NWT translates some verses a bit differently (but then so do some other non-JW Bible translations). And do some of these "23" also refer to the textual changes mentioned above? If is, then so have the other Trinitarian translations mentioned "change" these verses.