Hahaha, of course uncomfortable, 215/45 sidewall thickness or height is only 9.7cm. You should've just stayed on 16'. If want to improve a bit more comfort on your 17', you should use 205/50/R17, the sidewall is 10.25 cm, providing additional 0.5cm cushion compared to 215/45. But don't expect it to match your stock comfort. 215/50/R17 is nice but rather too big compared to your stock diameter.

Like I said above, it's a bit too big, so not really recommended. Better use 205/50/17.

Either you get used to it, or you get tired of it. I was the later one.

Again, a bit more comfy since the sidewall is almost 0.5cm thicker, almost the same height as the 205/50.

Yes, only a little bit.

Yeap, almost 1cm smaller diameter.

What car? If FC is a bit higher that means you're not using lightweight rims.
Lowering your car with sport springs will much improve stability. Strut bar though not so much unless you go racing or jumping up and down in rally tracks. Better spend the money for better dampers and anti-roll bars, they will give much more significant improvement. I'd think 215/50/17 should still be ok though, I'm also using the same size for my front.

Anti-roll bar or sway bar is a bar that tie up your left & right suspension underneath to make them move in a more synchronized/even manner. It basically reduces the car body roll/lean angle during cornering, hence makes it more flat, and makes your handling sharper and gives better grip in the corners. This you can feel more improvement for street use. It has some flexibility depending on the design and thickness. The thicker the bar, the less body roll you will have, but it may also make the car more nervous and stiff when going thru uneven surfaces.

Strut bar on the other hand is just to tie up the left & right top mounting points of the shock/strut. So the purpose is just to increase chassis rigidity/reduce chassis flexing. This chassis flexing becomes significant during very hard maneuvering like hitting the apex curb during track racing, or when going fast and jumping on uneven surfaces like in rallying. For street use cars, IMHO this is not much needed because you won't be doing such extreme maneuvering which will cause significant chassis flexing, hence even when you install it, you won't feel much difference/improvement.

Can be the tires & rims not well balanced, can be your engine and chassis already cannot cope and creating excessive vibrations. Tires even though cheap but if balanced well and no bulging then should not vibrate. Balancing problems normally happens only at certain speed range and does not happen below or over that speed range. If the vibration just keep increasing as the speed climbing and never goes away afterwards, then most likely not balancing problem.

Sure, why not?? On wheel balancing is with engine off and transmission in neutral anyway.