Your explore of the old tight adit was interesting.
Did you note any interesting artefacts in the shaft, pump rods or whatever. I saw the rising main, so presumably it was an engine shaft and your entry the drainage adit.
I would say that the adit was driven as a crosscut, and the sharp turn where it met the lode. The arching was probably to form the drainage adit with either loose ground or maybe even a stope above. So if you could have got up the shaft somehow, you may have got into workings. The shaft being sunk on the lode which appears vertical or near vertical.
Any sign that anyone else had been in?
It reminded me of one I did down Cornwall. I could actually drive my car down to it, and a crowd gathered while I was getting changed and watched me go in.
I think they were taking bets about whether i would come out again!
By the way, timber that below water is usually sound. Also in time it gets pickled with the dissolved mine salts in the water, then it never rot. In an LED light when sawn it looks blue!
My avatar is a poor likeness.