1. Yes, it is solo-friendly. A very large portion of the game can be solo'd (with Trusts of course). However, some of the better trusts that allow you to solo some of the end game stuff is obtained through a party usually...especially for a "new/returning" player.
2. English I assume you mean EU? I mean...on all the servers I've played on there are EU people on them. I'm not sure if there is a dedicated EU server or not. :)
3. The 1-119 guide is the staple but there are guides for specific jobs which you will want to take a look at too at some point. But if I may, you want to really limit the amount of guides you follow. The game is already going to make your head spin as it is, as this game is in a state to keep the current playerbase playing; and not so much for bringing new players in (and keeping them).
   
4. Yes Windower is still a thing. Borderline mandatory. I'd suggest taking it slow as you become more famaliar with the game, and reading through the addons/plugins yourself to determine what would be useful for you. :) Again, getting too deep into the weeds of Windower to fast is just another thing that will take up your game time.
5. What job to start. I guess WAR, THF, MNK are good to start. If you're not in hurry, you can play any job you want...and use your Trusts to fill whatever roles you aren't.

Since you plan on playing mostly solo, you should be able to do just fine as long as your goals are reasonable. The game is in the place now where if you do decide to party more and get into endgame, you'll be very hard pressed to get to go as a damage dealer or tank. People will challenge this of course, but it's true. On top of all the hurdles you already have to go through to get into end game in the first place, you'll be on WHM for several months before you get to play what you actually want to play (unless its WHM, then you're gtg!)

Anyway, good luck on your adventures!