I believe you. Just have never seen any official mention of 20K being required for the visa exemption, rather than 10K. I've always carried 20K, just in case. More is better than not enough.
If you don't meet the requirements and don't get the extension, and you do it on the last day, you'll get a "7-day to leave" stamp. But it's a tourist visa extension. The likelihood of being denied are almost not possible. Going earlier or later will have no effect on the chance of getting a visa or not.
Not going on the last day is simply better for you, because if you're missing a document, or your transportation breaks down, etc., you won't end up on overstay, which isn't good at all. Better to go a week or so before.
1. No, you can't do the extension as soon as you arrive. Go a week or so before the day on the stamp. Check this group to make sure you don't go on a holiday.
Correction first: SETV (Single-Entry Tourist visa) is valid for 90 days, as in you can enter the country within those 90 days. You don't get 90 days and extend for another 90. You only get 60 days, and you can extend for another 30 days at immigration for 1,900฿.
Your parents could apply for the METV (Multi-Entry Tourist visa), which if timed right, could give them 9 months in total. Although, they would still need to meet the financial requirements.
(Keep in mind that, depending on your parent's health, no matter what visa they get, if they want to stay 4-5 months, they will still have to leave the country to do a border run, which can be physically demanding and complicated.)
A good solution would be to get a SETV in Canada, extend it in Thailand (3 months total), leave the country and come back in on a visa-exemption stamp and extend it (45+30 days). That gives them over 5 months.
