First of all, sorry for your fish loss; even after a bunch of years I still hate losing a fish unless it is sick or old age.
I believe in KISS, (Keep It Simple Roy), and here is what I would do. First, I think you did the right thing by getting the bubble counter out of the system, you can always add it back in after everything is stable. Second, it is difficult to tell if the problem is the regulator, solenoid, or the diffuser. Let's put the diffuser off to the side for a few days as well.
Now for a quick checklist:
1) Open the valve fully on your 5 gallon CO2 tank.
2) If the regulator is adjustable, adjust your regulator output to about 20 PSI, if it is not adjustable, hopefully it is about 20 PSI.
3) Activate the solenoid so the light is on.
4) Adjust the needle valve (with tubing attached and going into a bottle or jar) so you are getting about 5 bubbles per 15 seconds from the end of the tube in the bottle. Give it a few minutes to stablize and check the count again.
5) Turn off/unplug the solenoid and verify the bubbles stop after a few minutes.
6) Turn on/plug in the solenoic and verify the bubbles start up again.
7) Repeat steps 5 and 6 several times until you are confident that the solenoid is turning off and on the CO2 flow and that the bubble count remains stable.
Then turn the solenoid off, let everything cool down, and turn the solenoid on....is the count still more or less accurate?
9) Once you are confident that the regulator, solenoid, and needle valve are operating properly and are stable, then try the system with the diffuser attached. Keep in mind with the diffuser it will not start immediately when the solenoid starts the flow because it takes a while for the pressure to build up sufficiently for the bubbles to be forced through the diffuser nor will the diffuser stop immediately with the solenoid stops the flow because it will take a while for the pressure to drop back down.
Hope this helps, let us know how things go.