I BELIEVE, you self police yourself and self isolate if you can for a few days, if you have mild or no symptoms. At this point, I believe the day 5 reporting is for statistical purposes.
Technically, if you split up your first 2 months with a side trip somewhere, or extend, you wouldn't need a visa as both countries (Definitely UK) are visa exempt for 30 days in Thailand. And UK doesn't need one for Bali either (And I believe Canada is the same).
Yes, but if you are vaccinated, you do not need covid tests to transit Tokyo, or enter Canada. The OP is transiting through Tokyo, and entering Canada. (I don't know for unvaxed transitting though as far as transit through Tokyo.)
I will be doing this flight in a few weeks. As of now, no testing at all needed if you are transitting. Japan have just changed the rules for anyone actually entering Japan, but transit at the same airport (not getting luggage, not passing immigration etc) is all good.
Take your Thai gf/wife/partner with you, and let her do the talking. That greatly enhances your chances. You will need to show your passport/visa, and I was also asked for the foreign bank account most of my funds would be coming from.
Are you sure it just wasn't a change of fare? You generally will have to pay the fare difference. And the closer to the travel date, ticket prices will usually only go up. And different days can be different prices.
Yeah, you will definitely want Baht in your pocket, no matter how you get it. Most tourists exchange their Dollar/pounds/euro etc as soon as they can after arriving. Save the credit cards for larger purchases, or emergency withdrawals.