1. Facing is an optional rule. Without that, or a similar rule, in place, facing doesn't matter.
2. Blindsight does mean you are "seen." It is blindSIGHT.
I run a game for kids. Just remind them to buy healing potions when in town and make sure they happen to find them as the need arises. No other changes necessary.
I've been in groups that have done it occasionally for various reasons, but it's not ideal. It definitely limits roleplay when you have more than 1 character per player.
Don't do anything. No DMPC, no sidekicks. Just deal with 2 PCs. It's perfectly fine like that. I regularly run for just 1 PC. Sometimes there are NPC allies helping them, that's up to the PC. But I never plan or worry about more than the one.