Bad 8GB RAM on 2013 Macbook Air 13" - had I bought one with 16GB of RAM, is there a chance that it could still be running on 8GB of RAM?

Always a possibility. If that doesn't happen in the first 90 days of use, then it is very unlikely it will ever happen. But no one can tell you with certainty that a memory chip will never fail.

Most likely Apple's reasoning is to keep amateurs from screwing up their computers which Apple then has to fix or replace. It also encourages buyers to pay more money for their computer and keep the computer longer.

Memory tends to be fairly robust, but the biggest danger is probably electro-static discharge completely killing a module after being handled. It's well known that Apple notebook computers with user-replacement memory can operate off of a single module, but that might not be possible if there's some sort of socket damage that registers as a memory error rather than just unpopulated.

They do register as two logical modules. I don't know if going for more is going to help if one logical module fails because a memory module that fails will likely have some sort of error (that doesn't get past power on self test or ends up as a random panic if it does boot) rather than just register as unpopulated.