It will most likely rain at some point but it's not going to be monsoon style rain. It might rain for an hour or so then the sun will be back out. Likewise there will be days where there's no rain. Very unlikely that you'll have an entire day washed out.
2) if at university yes, if doing non-university course then it's much harder and unlikely
3) yes but you need to be attending class in Thailand or your student visa will be cancelled by the institution. Unless you have a dodgy visa of course...
No. You might be able to get a covid extension if you apply before 24 May but short of extending it for the reason you got the visa you don't have many choices.