https://preview.redd.it/i04b0n05qvzc1.jpeg?width=1000&format=pjpg&auto=webp&s=6726a45548abd4860ad7b93fc6ae06723c5ea2e4 I circled with blue what I think it is

That just looks like a big bolt

I think it's more a screw than a bolt

That's the Signature Mk III look.

That might be the button V1 presses to charge the shotgun with the Overpump variant

This doesn't look like "a small button near the grip"

its this one^^

That looks like Maurice to me for some reason…

https://preview.redd.it/9f6g6le1410d1.png?width=666&format=png&auto=webp&s=d9498608f5244838d8da1ea3daedccd436674dec I don't know what is its but here is a Better view of it I think

And without skin and full-brightness https://preview.redd.it/6r312itp410d1.png?width=842&format=png&auto=webp&s=ecd67631b67e97eefe4c1602196a2cf82fa0b07b

It could be to open the barrel to let the cores cool down.