Regarding the land borders, Myanmar, Laos & Cambodia are all reporting less than 200 cases with only 6 deaths in the first. With over 3,000 cases it's Thailand that they are defending themselves against. The decision to reopen will be down to them methinks.
your no.2 emphasises the word before. Is it not possible to purchase the 60 day extension during the amnesty period? In other words, a 90 day period has run out, and the visa holder is reliant upon the amnesty. Can they not buy the 60 days? A theoretical question and asking for a friend.