Gloss and as you say "wet look" mostly comes from preparing the paint for waxes or sealants. OCW and OID will give higher gloss than opti seal but the difference will be minor.
If you really want to maximise your paints gloss you have to wash it, de-contaminate (remove iron particles, tar particles, clay it) and polish it (I suggest hyper polish by machine or poli-seal by hand if you dont own a polisher). This will give you what you are after. Whatever wax or seal you use later will just preserve that finish.
And to answer your questions:
- Yes you can apply OID on top of OCW. Just give it an hour before you do.
- OID will give more gloss but OCW will be more durable.
- Yes you can use either OID, OCW or seal after every wash. Seal is kinda my favorite rinse aid.