Re: Shower Faucet Issues
It could be the anti-scald device, which is little more than a stop that prevents the hot side from being turned on too far. Readjusting the stop make take care of the lack of hot/long warm up problem.
As dj suggests, there could also be a partial blockage in the lines supplying the valve. Turn off the water main and disassemble the valve again. Now, turn on the water for a second and back off. If there was anything stuck in behind the cartridge or valve, it should have blown itself out with this maneuver. You'll probably want to have a help, one person to work the water main, the other to monitor the valve and keep water from splashing and making a mess.
As for the shower head, you might try removing it once everything else has been done. Turn on the water to blow that line clear as well as check the shower head itself for obstructions or deposits.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!