Hi @Carreta
I only skimmed a great deal of the replies you got, so I am likely to duplicate some.
I suffered a fairly severe neck injury at age 17 while on a motocross motorcycle. Basically, I went over a hill and landed on my back, jarring my neck out of place. I was doing this against my parental instruction, so I never reported the problem. I suffered with not being able to tilt my head upward at all, chronic pain, and not really look left or right without discomfort.
This lasted until I had a son-in-law finishing up Chiropractic school. He put my neck back into place and had me see a local chiropractor and massage therapist for a year. I have been in pretty good shape since then ( over 25 years now) with reasonable mobility and few needs to get my “head put on straight” again.
What I learned from my time BEFORE the chiropractor is that maintaining your shoulder and back posture is critical. I learned this empirically. I sit in chairs VERY straight and I pull my head and shoulders back so I have no hunch in my upper back area.
It helps to have anoter person let you know if you are out of good posture. I am married, so… 
What you are going for is to have your shoulders look flat across the back. Look up “scapula”. You want to actively maintain a little pull to keep them together rather than falling forward.
You want to try to keep your chest up. This will help help your mid back in the right position and also push your head back a bit. Think of a military person standing at attention. Your second picture shows a pretty rounded mid back.
Lastly, keep your head BACK. that first picture has your head bent down, creating tension in your back and neck. If you need to look over your guitar, do it for a shor time and position your head correctly again. This may feel like you are out of balance, but you want to keep your chin a bit down (not a lot!) and then balance your head so you don’t need a lot of effort to hold it in place.
If you can get into this habit, you will benefit yourself a lot as you age.
As for your seat, I think a moderately firm seat is better than a soft one. It doesn’t need to be fancy. I use an office chair that was too large to fit in my desk space. It is capable of setting a firm back, tilt for hips, height, and no arm rests (removed). I rarely need to touch the back if I stick to my instructions above. I have successfully used a footstool and a simple diningroom chair as well.