I have a Vela semi and an Epiphone SG. When sitting with the Vela (with the guitar on my right leg) it's shifted way to the right making access to the upper frets difficult. When rested on my left leg it's much better. Playing the Vela while standing is pretty comfortable but things are still shifted to the right a bit more than I prefer.
Keep in mind that I have some mild problems with my fretting hand and I'm not downgrading the Vela at all. I've said it here before, of all the guitars I own the Vela took me the longest to adapt to because of hand issues. But playing the Vela while standing is very comfortable once my hands are warmed up. I think because of the Pattern Regular neck carve being a bit on the smaller side I found I have to think ahead more on this guitar than any other especially when changing fretting positions from near the nut to the 12th fret. It's like I'm telling myself "don't forget to place your hand like this when playing that chord at the 12th fret." I've never had any shoulder issues as you mentioned with the Vela.
Again, because of hand issues, my SG is more comfortable while sitting (with the guitar resting on my right leg) than the Vela because everything on the SG is shifted way more to the left. For upper fret access I haven't played a more immediately comfortable guitar sitting or standing. But again, that's what works for me and may not apply to others. We're all built a little different and what works for some may not be so good for others.