RE knocking pressure on the hot water.
Why would someone suggest placing a pressure reducing valve on the main (C/W) if this phenomena is only happening in one bathroom and only on the hot water?
Seems to me if it was strictly a "pressure" problem it would be system wide and not just H/W huh?
Loud knocking sounds could be contributed to long unsupported runs of tubing / piping
failure to allow for expansion and contracting of hot water lines.
Something as simple as lose washed on a (tile stop) shut off valve that controls the shower and basin
Many times an installer wants to cut costs and does not use a four fitting swing to allow for expansion and contraction of piping movement.
If this was a hydraulic shock wave from excessive pressure it would stand to reason that when a toilet is flushed or a solenoid valve closes from a dishwasher or washing machine you would also hear the THUD which this is not the case so FORGETABOUT a pressure reducing valve as you would be adding more mechanical devices to go wrong.
Try tracing the lines to this one bathroom and make sure they are secured properly BAND IRON is not a proper support clevis hangers are the way to go.
Make sure the piping is strapped with a lie wise material to prevent galvanic or electrolytic action of dissimilar metals ferrous and non ferrous.
Shut off the supply valve to this guest room and open the valve partially as this is what a globe type of valve is supposed to do THROTTLE the water flowing though the valve.
As you slowly open it have someone run the water and see if throttling down helps.
Air chambers are on as long as you have a way to replenish the chamber and are properly sized and even they are not going to stop a chattering bib washer or piping improperly installed.
Compression type faucets have a bib washer and a lose washer sounds like machine gun firing with lots of vibration.
Try to keep it simple, do not start complicating the job by adding more devices for the sake of guessing.
Has this problem recently started of was it there from day one?
Did anyone add or change anything to this room?
Trouble shooting is not replacing or adding things to the mix Isolate then approach it logically.
If advice does not sound right then it normally isn't...