The '96 Force 4 cyl. has the CDM module coils. No bench test for them, and if your mechanic had some "laying around" then they are probably bad. Merc has only "improved" them (and superceded the part#) about 14 times. I would gladly take it as a new patient, or I could give the mechanic closer to you some tips on getting to the true issue.....since it sounds like he is chasing his tail and/or has missed a few "service bulletins" on the CDM ignition system.
If it is truly an ignition problem then he needs to verify stator and trigger output, if they are within spec then replace all 4 CDM modules (coils) with THESE...
http://www.marineengine.com/newparts/part_details.php?pnum=CDI114-7509
Do not replace with any of the Mercury #827509A1, A3, A4, A7,A9, A10, or T7
Test the voltage regulator.
And be sure the correct spark plugs is used......which should be a Champion QL76V and not the NGK BUZHW that is in almost every 4cylinder Force that comes in here... because that's what happens when people shop for spark plugs at automotive outlets.