No.
The time it sets could be off by less/more than +/- 30 seconds from the real time everyone else is using.
Not maintained by me.
If you know how to do that generally, you might know how to do that for Whonix too. Self Support First Policy for Whonix applies.
Either gateway or workstation. That depends on the setup. Adding extra network adapter to workstation might be easier. That other VM might then exclusively grant access to a database server to that workstation.
I haven’t tested that.
Not sure the current configuration ability is sufficient or if script / iptables rules modifications would be required.
I haven’t seen anyone online saying doing that yet.
There are pointers here - Onion Services - Whonix - but not, no research into scalable onion services has been done by me, let alone how to integrate that with Whonix. Non-trivial to say the least.
Cannot see how.