Torn Sinew is available past the end of the current campaign. You can’t upgrade items past level 12 until then (though depending on your character level might see drops up to level 14).
You may have seen OP builds from before the most recent patch, which has drastically reduced player power across the board. This was slightly controversial, but in my opinion enormously healthy for the game. (And you can still get OP in post-game if you want.)
For UI elements of unknown meaning, a screenshot is worth a hundred words.
The ‘mysterious money glitch’ has been confusing people for a good while. Coins are converted to higher denominations: Once you exceed 100 copper, they’ll be converted to 1 silver, and 100 silver to 1 gold. Nothing is bugged; internally money is calculated in copper coins.
If you’re at 87 copper and sell something for 30 copper, you’ll be at 1 silver and 17 copper.
Compare, for instance, this compilation of reports: