Can moen L-2351 be used just for shower instalation

SOURCE: Moen shower head drips. Single lever shower only.

Probably need to remove and replace the cartridge. Often sediment will tear part of the o-ring or seal on the cartridge and cause this problem...
varies by model, but generally, remove the cap on the knob, unscrew the knob, remove the keyed sleeve, remove the round trim plate & shut off the service stops (verify they are fully closed by pulling out on the valve stem) next remove the retaining clip (needle nose and be gentle) that holds the cartridge in place, then pull the cartridge straight out with vice grips or similar (or get a puller). clean the cartridge, flush out the valve body by slightly opening both service stops, reinstall cartridge and clip, open service stops and see if it still leaks. if it does, take your cartridge to the store (any reasonable HW or plumbing store should have a replacement) and get a new one. then reverse the process to reinstall.
If you send me the model I'll get you more specific instrucitons and pictures...

SOURCE: my moen shower valve didn't come with any

it is used for keeping the valve from coming thru the big hole when you tighten up the main cover,...it's to go on the back of the shower wall,..you don't have to use it if your valve is secured enough via pipe clamps and/or the like. good luck

SOURCE: flow restrictors

Remove the shower head from the arm and look inside the connector on the showerhead. You should see a plastic or metal piece that will either unscrew or need to be pulled out with needle nose pliers. Once that's been removed the flow should be improved.

SOURCE: moen monticello shower control, remove handle

If your Posi-Temp has no water coming through the valve my first step would to replace the cartridge (Moen pn 1222). The valve as a simple one. Get the part first so you can ues the plastic tool that comes with it if you need to. Beneath the actual lever insert on the handle hub you will notice a small hole. this is to gain access to the allen head screw which holds the handle in place

SOURCE: Had a leaky Moen Posi-Temp shower faucet.

I'm not aware of any problems with non moen replacement parts being the cause for a water hammer problem. Start with the simplest and cheapest correction which is using a moen replacement cartridge. If the water hammer persists then you may have to install a water hammer arrester in the plumbing system. This is basically a shock absorber in the plumbing system to stop the effect of water hammer. An arrester will cost you around $10 to $15 at your local plumbing supply shop. If you are not comfortable soldering copper then you will want to hire a licensed plumber to complete the work for you.

I had the same type of problem with an older toilet in my house. It developed a water hammer when it shut off after filling the tank. I replaced the fill valve with a generic universal replacement for about $10 and it stopped. Again sometimes the simplest fix will work. Let me know how it works out.