I’ve used e-transfer, PayPal, and Wise (formerly TransferWise) and they all work. I’m a bigger fan if wise since the conversion fees are the lowest.
You should not charge tax since its a US company and you don’t owe hst on that work (still need to for income tax though obviously).
Haven’t used a business account before so I can’t speak to the pros and cons of that approach
Dowels work too if you’ve got them. I use that method for concrete too, since I find tapcons frequently fail to grab. Drill out the concrete hole larger than the screw. Shove a dowel or a bunch of toothpicks/splinters into the hole, then drive the tapcon or a wood screw in. Holds like crazy since the wood conforms to the concrete wall imperfections. Might rot over time though. Maybe plastic is better.
Check out RJ the bike guy on YouTube for great repair videos for old bikes.
Here’s an example if you need to change out the calipers with a different mounting system.
If you’re more into YouTube video tutorials, check out RJ the bike guy