My price list is a few years out of date but I will PM it to you if you are interested. Buttons are sold by the gross, 25% upcharge on orders less than a gross per size, style and color.
When seated, the shoulders can hunch forward and can cause the shoulder to twist, as is the case in the first photo posted. When standing naturally, if the twist occurs on one or both sides, it is because your shoulders are pitched more forward than was accounted for when making the pattern- this can occasionally be fixed by a tailor but it's not cheap. You will usually feel pressure on the shoulder point as well.
You are seriously considering building up your arms only just so you can still fit in to your suit? Oh. My. God.Yes it does- I have close to 40 suits that I can't wear anymore. I used to be able to fit into off-the-rack sizes fairly easily but not at all now; if you are serious about lifting, you had better get well acquainted with an MTM or custom suit maker.
Can't say without knowing how they will take the excess out of the waist.It's very tricky to do on finished pants- talk to your tailor about whether he would want to attempt this before making that kind of a change. Don't touch the thigh- it's far easier to reduce it than to try to let it out if it ends up too tight.
Adding a dart without increasing the seat measurement will not help this- you need to increase the seat, which is too tight. The point of the extra dart is that when increasing the seat measurement on the pattern, you will also increase the waist- the extra dart allows you to remove that additional width in the waist but leave the ease in the seat. I could add a drawing if that would help you understand.