Within the last month I have started getting only lukewarm at best water from my shower.
The shower is a one handle moen? turn right for cold and turn left for warm. All the way to the left is now luke warm at best. Even if I let it run for 5min or so it is still barely warm. I did replace the shower head but it is still lukewarm at best.
I have drained the water heater (50gal) but had an issue doing it. I closed the incoming feed to the water heater and then opened the faucet at the bottom of the heater but it stopped draining within 10min. When I opened the emergency pressure valve it started draining again from the faucet for a few minutes. I did this a few times and each time water would drain from the heater. I would have expected that it would have taken a while for a 50gal water heater to fully drain.
All other faucets in the house get very hot water.
Another note is that when I turned on the shower after closing the feed to the water heater I got no hot AND no cold water coming from the shower.
All other faucets worked fine getting cold water but no hot water once the water heater was shut off from its water feed.
I have not taken apart the handle as of yet.
Right now I'm not sure if it probably the regulator in the shower handle or if I have a problem with the water heater itself. Certainly I would rather not replace the heater if possible. The heater is 7 years old.

You have a rather costly moen pressure balance château shower valve. Likely you simply only have to remove the handle and adjust the handle limit screw