If you’re planning to stay in Thailand more than 90 days but less than 120 days total, then you don’t need to bother with getting the 30 day extension. Just leave before your 60 days are up and then you’ll get another 60 days when you come back. That’ll save you the hassle and cost of extending, and as others have mentioned might make it less likely you’ll be questioned upon re-entry to Thailand (but still be prepared for the possibility).
No official limit, but would be better if there was one- maybe no limit on entries, but maximum 180 days per calendar year on visa exemptions. That way everyone would know the rules and would ensure it wasn’t possible to live in Thailand without a longer term visa.
Not at this moment and there’s yet be an announcement about exactly when this might change. Same for retirement and elite visa holders among others. Likely not until September/October at the earliest.