Most real financial transactions need to start and end in fiat though, and that's where the costs are

There are pretty big markets in these countries because of crypto and even then it never reaches the native currency a lot of the market uses either us dollar or euro. Businesses are glad to accept dollars and euros for most bigger purchases especially p2p marketplaces. The myth that you have to off-ramp crypto is no longer true.

That depends though. I recently bought a car in south America and they were willing to take USDT through Bianance Pay. As a US person I don't have access to that so ended up having to wire transfer. I don't know why they wouldn't accept a Bitcoin deposit which they could have easily converted to tether but I suspect it's just a momentum and learning curve thing.