I wouldn't buy a new cartridge but since the other one seems stuck in the valve body due to corrosion and what-not I'm going to have to tap and use a bolt to get it out, which will ruin it. So, there is no way of just spinning it around. I tried today, and since it's plastic I didn't want to put too much force on it and risk breaking it and NOT have a shower.
As for the stop-check valve, since one is seized in a partially closed position and it's got to come out anyway it'll probably need replacing. I figure that since I'm going that route then I might as well replace everything that could go bad or is bad. I did try to get the stop-check to turn but I broke one side of the screw that protrudes out of it, and rather risk breaking it to where it leaks I decided I had better get one to have on hand. Worst case is I replace the old ones with the new ones, clean the old ones up so they are operable again and keep them as spares.
I'm far from a know it all. Hell, I'll be the first one to admit that I've got absolutely no frickin' clue what I'm doing; that's why I'm here.
I know ultimately, I need a water softener (dunno why I always forget that "e") but in the meantime I need what I have to work. I've been living with poor water pressure in the tub/shower for a long time; it just took the wife to start complaining about it before I decided to attempt to fix it. The water softener is coming, soon as I figure out if a neutralizer is needed or not. I'm going to drop a sample of water off at the plumbing supply house where my wife's ex works so he can get it tested this way I can have some more numbers to work with other than the 10 month old ones I got over last summer.