Lots of good questions. Here are some of my shortcuts:
The B18 using a "91 distributor" (Should I assume a SOHC Distributor that came with the car?) would need the distributor to be rotated further than the slots on the B18 head allows. there are kits and the like to do this.
You may be seeing where it's not advanced enough to run real well. But don't go grinding your engine yet !
The ECU in the 91 chassis is pre OBD, and sits on the floor. Any of the OBD ecus that will talk to the B18 won't be that kind of form factor. They are normally zip-tied to the firewall. By the sounds of that connector, you're still pre OBD.
Hondas are great because if you simply remain with the ENGINES ECU, and not the BODY'S ECU, the engine will run fine. Not that you can't modify the engine / plugs / ecu to run, but is it worth it (Or does it fit into your technical abilities?)
Also with a swap, especially one that's been done by someone else, it's not possible to confidently identify the part's year or vintage by the usual connector / markings. I've built hybrid distributors out of many distributors (with OBD pickups, SOHC housings and DOHC rotor shafts, for instance) so looking around in my garage will make you feel really impotent. So many things are interchanged or interchangeable that it's silly.
The best thing to do here is keep the hardware together. Use the new distributor for the year of engine, and modify the connector to line up the wires (Simple, constant power, triggered power (Pulsed power) and Tach wire). The ECU will know what to do with the spark (Because it's still a 4cylinder) , Also remember that in this setup the Tach will see a problem at the same time the ECU does. the Tach works, the ECU should work.
