1. PostgreSQL. Fast, reliable, (relatively) small (for some reason mysql got a lot bigger). Had a few historic problems with replication but I think that's been resolved. I dealt a lot with large scale large data sets and I wouldn't recommend mysql for that at all, except if they were very simple and relations were either not used or used minimally. There are better database systems than either, but I can't think of a less reliable one than MySQL. If you use PHP though, mysql is easier.

(I'm going to assume the rest of the questions are SQL related and not SQLite, which is a particular binary format)

2. why? SQL + json would make some sense, sometimes. I haven't needed it - I just do SQL. Others have better answers.

3. There are tools to do some XML to and from SQL. YMMV. I always rolled my own.

4. it's always possible. Is it worth it? I used CSV with embedded json for a long time with archival storage because it was reversably parseable and could scale time-based databases somewhat reasonably. There are smarter ways to do this too ...