As I had already mentioned, "Also in that 2nd picture, they apparently disconnected that now capped line dropping from above.
Unless you know where that capped line continues up to, we don't know if that was a drain line or an auxiliary/relief vent from that large horizontal main line."
You're calling it a, "stack vent line". Do you realy know what its function was? I don't.
Well I am not a professional plumber. You might say I dabbled in municipal and industrial plumbing design and engineering, for a number of years.
As far as, how the supply lines were installed. Looks like sh*t. Don't know why they bothered using a box with drain connection and not use it. But I suppose it serves the purpose of mounting of the wash valves.