Sounds like your pretty handy if you've had the cartridge out already....nice...and a gutsy move to turn the water back on with the cartridge out. Now let's move to a solution. Take your shower head off with the arm still in place and the end open. House water off, use a pipe cleaner, (the kind of pipe you smoke dude), and insert it into the seat orifices that should be visible with the cartridge out. Run that pipe cleaner up as far as you can and work it back and fourth.
I would imagine if there is no water coming out at all when the water is on and the cartridge is out....you have a very tightly packed slug of debris. Likely spot for such a plug to develop is at the smallest port in the system, which is right there before your eyes, so if you felt like your pipe cleaners may have punched through a clod of debris, go out and turn on your water service, but just crack it open a little.
Go back in and check for signs of water. If a little water is showing now, open a faucet to release any possible built up pressure in the system, then go back and drive the pipe cleaner back into the valve seat orifices to break up the debris some more, until it flows freely.
If you still have no water flow after using the pipe cleaners, go out and crack the main water valve slightly on, again open a faucet to relieve any pressure, (good idea to remove the aerator on the faucet before starting this process).
Now use your allen wrench set to probe the orifices. the L shaped individual piece set is the best type to use...now begin with a very small allen wrench and twist and spin it to see if you can dislodge any debris. I sometimes use the short end of the wrench to turn a corner on a hole if it takes a turn before exiting the orifice hole.
If you think you dislodged some crud, go turn off the slow running faucet and water should begin to appear from the holes, if so continue to work the allen wrench in the holes.
Step up a size at a time until it becomes too big to fit the hole. Finish the clearing with that pipe cleaner or until no more debris are spitting out of the holes.
I suggest turning the volume up slightly at the main shut off valve, increasing the flow through the holes.
Reassemble the shower valve and turn the faucet off on the sink. Flush the shower valve by opening the valve full on and full off , rapidly, a couple of times. Work the shower valve in every way it can be moved.
Look for further debris exiting through the shower head arm now. When all looks clear on the water flow, go flush out your sink faucet in the same manner as you did the shower valve, then replace the aerator on your sink faucet and replace your shower head on to the shower head arm.
Good luck with your problem and don't forget to turn the main water shut off valve back on 100%, then back off just a tad from absolute full open position.
Flush all of your faucets and exterior hose bibs when you get a chance too....likely that debris have affected all if such a high volume is present in the shower valve.
If you still don't have any water flow after all of this effort, consider hiring a Plumber to delve any deeper into your orifice holes but it all might be futile at this juncture, because chances are you need to re-pipe your home.
You could start small by replacing the shower valve and supply piping, then piece it out fixture by fixture, but before starting a process like this, make sure you have a clear idea of how you plan to replace the pipe on every fixture, because if you run the supply pipes back up to the attic or down through the floor on the shower, it might not be the best direction to tie them back into the rest of the new piping on the next phase of the re-pipe. Capiche?
In a lot of cases, the majority of the new pipe on re-pipes will be done using cabinets or open walls down low, where it is easier to access than in the attic or under the house. Plumbers who do a lot of re-pipes will have an expert patch man to cover his tracks when the piping is all done.
Hope I didn't totally corn fuse you. but I have a feeling you might be smarter and handier than the average bird...

Moen products are the worst on the market. Remove the trim. Check that both stop valves are fully open. Then get both cartridges and replace them. Moen will warranty them, but you have to wait for delivery.