Where are you trying? Pattaya? If so you’re going to have issues. The issue with this group is that people don’t give enough information about their issues in order to help in many cases. They believe that everywhere in Thailand offers the same experience they’ve had and can’t accept that elsewhere may differ.
No issue. Bring both passports with you and show the officer your re-entry permit in the old passport. They’ll stamp you in on the new passport.
Then go to your local immigration office to get your extension stamp and re-entry (if multiple) placed in the new passport. Once done you can forget about having to need the old passport.
No one can say for sure as it’s up to the individual immigration officer you hand your passport to. But 86 days this year plus wanting more isn’t a typical tourist thing and may certainly raise red flags.
To CYA have the three proofs that they may ask for.
1. 20,000 baht (or equivalent) in cash.
2. Proof of accommodation (for the first few days at least)
3. A ticket out of Thailand within the 60 days allowed (you’d probably get away with 90 but your airline may cause issues).
Yes. A multiple re-entry permit allows you to leave/return as many times as you like for the validity period of the permit (your allowed to stay stamp).