Never thought of it like that but I guess yes, you could reuse addr's

It'd just show up in the mempool more when viewing confidential TX's

Yes, just post a liquid qr code as you wish. You have to generate it from an address which gives you the ability to use confidential transactions first, or else it won't work.

Confidential TX's are not automatic. I believe only running a liquid node allows for that rn. Idk any other wallet that does this