Yep! ICE lets you try a bunch of different ports and protocols.

So you can have a remote peer and try to contact it via UDP/TCP/TLS. You can even attempt via multiple interfaces (Wifi and 5G).

You can then measure the packet loss/latency across these different paths and figure out which one is best.

Likely yes, via TURN servers.

No. It doesn't allow anything that can't be done by a browser behind a firewall connecting to a regular web server outside the firewall.

WebRTC can create a NAT Mapping via STUN (UDP). https://webrtcforthecurious.com/docs/03-connecting/#nat-mapp... that is pretty powerful compared to Client/Server HTTP.

Dependent on the firewall, but most I have seen allow NAT mapping with different behaviors.

To my knowledge, most regular web servers either cannot speak DTLS at all or only have limited support for it, but all modern browsers work with it just fine for WebRTC.