1. He’d get 60 days on entry now with a SA passport.
2. In theory yes, but very rarely asked for at immigration on entry. More likely they’d be asked by the airline in order to board the plane. Some airlines are strict. Some don’t seem to care.
3. 20,000 baht in cash or equivalent in a major currency. Extremely rare to be asked for this but the rule does exist.
I’m assuming you’re coming from a country that allows visa exempt entries rather than having to buy an actual tourist visa, but as you don’t state your passport country it’s just a guess.
Your plans of entering Thailand shouldn’t cause any issues. You are doing what a typical tourist could do. Generally immigration are on the lookout for back to back entries on either visa exempt of tourist visas that could indicate that you are potentially abusing the entry system or potentially working here.
That said it’s up to an individual immigration officer to assess your entry so no one can accurately predict what they’d think at the time you give your passport to them.
You won’t be extending anything tomorrow. All immigration offices are shut because of the national holiday.
You may find a screenshot will suffice but almost certainly they will want a physical print of that screenshot. It really depends on the office you are going to use (on Tuesday) as what they’ll accept.
The airlines may ask you for a ticket out of Thailand within the number of days your entry scheme allows, regardless that you could possibly get an extension.
Extensions are not a god given right. They are at the discretion of the immigration office you apply at. Granted that probably 99.99% are given it’s not a guarantee that you’d get one hence the need for a ticket out within the number of days allowed.
Moderators on this forum know that the chances of being asked by immigration on entry for a flight or ticket out of Thailand is pretty much zero. Yes it’s a rule that you need one but they rarely ever enforce it and if they do so it’s because they are looking for a way to deny you entry - probably because of your previous entry history.
Airlines on the other hand can and do implement their policies if you don’t have an onward ticket. Some don’t. Some do. If you are one of the very very few people that have been asked by immigration for an onward ticket then it’s almost certainly that you had questionable previous entries.
The 60 day with visa exempt only started on the 15th July, so last time you entered you got 30 days. Now you’d get 60 so there is no point applying for a tourist visa.
You can extend one time for another 30 days at a local immigration office for 1,900 baht.