They have hundreds of models and the only way to find out which ones have problems is to test the specific model
Unfortunately no, it requires an investor, because it is a huge number of hours, and the number of buyers, unfortunately, cannot justify these costs now :(
Yes, it's more likely than you think)
This event occurs when a transaction is created on the blockchain but has no confirmations
This forum has BTCPayServer 2.1.1 installed and everything works. Also, your changes concern XenForo specifically, so the issue is probably in a specific XF version :)
Yes, you can make a payment using any cryptocurrency in the FixedFloat tab