My old washer dumps into a 1 1/2" standpipe then through a p trap and into the wall and down to a 4" main in the basement crawl space (accessible). The adjacent laundry sink tees into the same standpipe (above the trap of course).

Due to a sink change, we are now out of code and I want to upgrade the standpipe and trap to 2" for flow safety.

Won't dare go into the wall to replace the 1 1/2 in there -- old plumbing.

I would love to just drop the standpipe through the floor into a p trap (yep, keeping less than 48" long) and the n horizontal before turning down 3 more feet to the 4" main.

First question is how long must my horizontal be to AVOID the p trap becomming an S trap which is not legal in NJ.

Second question depends on the answer above. Assuming some horizonal length can keep the P a P, then I can run the piping two ways. (1) down to the 4" main Teeing into the main in a 2" T (which currently has the 1 1/2 old pipe in. OR (2) replace the 1 3/4 in the crawl space from the main all the way back to the floor above, and T the new standpipe (after the P).

The second question is whether I am close enough to the vent to be legal in (1) or (2) above. Does a vent have to be right above the T where the lines merge, or is the vent a vent for a line if it is within some number of feet from the merge?

A little guidance would help here.

Thanks