Dear Brihji,
On concentration, you are right - but can't comment much on how to improve as am severely a laggard there myself.

The only two things I can concentrate on are better performed inside the bedroom and best if outside in the open - in the divinely inspiring environs of Mother Nature. One of those two is reading a book.

That is where I think the mental strength training ought to come into picture. The quest is on...
Wrt to the posture -
1. REMEMBER ->
THE GUN IS TO BE POINTED DOWN THE RANGE AT ALL THE TIMES - so your option to fold the pistol arm is ruled out. Should anyone even attempt to do otherwise (s)she shall be disqualified. No stands allowed either - like those AR shooters.
2. What can be done is that the AP can be put down on the table (
if it's not loaded) with action open and you can do whatever you wanna do to relax. Sit, drink, eat, do pranayam etc. etc. In this case it helps to take a break every now and then - hence the 3-shot cycle (as I once did) or the 4-shot cycle (as Jitu does) or the 5-shot cycle (as I do now) followed by most shooters. You can massage your palms and blow air as advised by tirpassion. However, if the AP is loaded and you feel the pain or the fatigue - do whatever to get that shot off and only then you can put the AP down and break off. Ah! And how do you put down the case ? It should be put down in such a fashion that the gun inside is also pointing down the range and when you open it and take out the gun, it's in same fashion!!! I was very kindly informed of this "rule" at Capt EZ at MRA by a good guy there last year. Being a right handed person, I had put the case on my rhs with hinges to the left and the latches to the right - so the AP inside was naturally pointing towards me!!!
JFK's assassin - the Great Magic Bullet - must have made everyone extra cautious since.
3. Many shooters in between the shots rest the pistol on their pistol case or on the pulley machine. That takes considerable stress off the arm. But for taller guys like me (6'+) - that posture also tends to induce newer strain in the waist, the lumbar, the neck and the knee. It also spoils the stance, especially the leg placement. That pain is further accentuated if the shooter is endowed with "party-pack" abs-belly like mine.
Guruji and Upacharya et al - Hope my thoughts above are right. Now almost being 2330 of a Saturday night, I have to get going to do the two tasks that command my absolute concentration - shall start with reading .... till the kids are awake.
BTW,
Bon Voyage tirpassion - have a safe flight back to Paris. Update from me - done with filing in GFG and NRAI forms with the MRA finally. So unless "some house-fly sneezes" (as Marathi idiom goes), I will be at Pune in mid-September this year.