1.fly-for both reasons

By train: not possible all the way for now,as NO INTern. trains at all from GR, also not between other ex-Yugoslavija countries for now. Best way is use a ferry to somewhere in IT then train. Use a common EUR trainplanner,.f.e. from bahn.de or others to see what trains there are and how they are routed.

This topic has been posted so many times here on this forum recently-though mostly for visit TO GR-so just scroll down and read that for more info

OR search seat61, an /en/ site that overviews all overland connections to/fro UK and hook on somewhere convenient.

2.no-only part of the way and not continuous-mostly ITaly and parts of Germany. Or you could choose, if HS is wanted so much, even go via FR-the most KMs of HS in Europe. In general: there is very little to see out of trains on HS-soundwalls or beetroot/corn fields.

3.differ per country-also depend on where you come from and what countries you passed by the last weeks. For quick transit in most cases no test needed.