Best way to wet vent a washing machine and utility sink?
Q1: the code always allows a combo wye whenever a san tee is permitted. The reverse is not the case: vertical to horizontal turn must use a combo wye and may not use san t.

Q2. do not do that. code NEVER permits reducing pipe diameter in the direction of flow. So if the below grade pipe is 1.5 inch everything upstream from it must be 1.5 or smaller.

Q3: yes, san tee is permitted horizontal to vertical.

Q4: there's no need for a second vent merely because there is a high discharge volume. wet venting may be a problem, though. Also, the fact that the washer drain is 1.5 inch and not the code-required 2" is the bigger problem.