Question:
Can milk teeth fester and swelling (if it were pus) can swell quickly?
Answer:
Yes, milk teeth can fester. I can also explain the rapid subsidence of the severe swelling. Obviously it was a tooth abscess that burst (= fistula), which would explain the rapid decay. (Remote diagnostics are not possible, it would only be one option).
Answer:
As soon as the first milk teeth are there, start carefully and playfully, clean the teeth with a soft brush or cotton swab first.
Personally, I already use minimal toothpaste for my 14-month-old daughter (= barely visible on the brush), but of course she can’t rinse it out yet. Otherwise, I recommend a mild fluoride-containing pasta, not too sweet.