What's the correct laundry drain stack order?
You'll need to change the tee on the vertical pipe to a 2&quot; tee for the laundry drain.  You're not allowed to reduce to a 1-1/2&quot; for a few inches of horizontal branch and then increase to 2&quot; again for the washer trap &amp; standpipe.  However, the washer trap &amp; standpipe does need to be minimum 2&quot;.
PEX crimp connectors certainly can be behind drywall.  Check with your AHJ if you have any doubts about your local rules, but these are reliable connections.
You are probably going to need to reroute the copper supply pipes to fit the laundry trap in that wall cavity.  Keep in mind there are several height restrictions on laundry standpipes.  It must be &gt;= 18&quot; above the trap weir and 30&quot; to 48&quot; above the floor.  The trap itself should be &gt;= 12&quot; above the floor.