So would you call adjusting soluble boron "not adjusting chemistry?"
I call it a water add. I use either a computer program, some RO Curves& a procedure, or a controlled copy document called the RO Guidance that is provided to me by the Reactor Engineers to come up with an amount to add.
Most times I just add water for inventory and rod index control. Early in core life I have to make neutral batches instead of straight water to make up for inventory lost through the RCP seals because adding water alone would be too much from a reactivity standpoint. On one occasion the day after a misaligned rod incident, I was adding tons of boric acid to the makeup tank (chemical volume tank I think it is called for you Westinghouse types) to maintain rod index, and power constant due to Xenon burnup.
Boron is used because it is cheap and effective. I know chemistry is busy during startup adjusting Lithium concentrations and sampling because OPS is changing the Boron concentration a lot. The Li control is important to maintain a near neutral or slightly alkaline RCS chemistry. During normal plant operations, I will sometimes put a demin in service per chemistry request to remove lithium (made through boron activation).
As for the pressurizer, it is an important component in a PWR. It is what pressurizes the RCS. It is especially important during a transient because it acts like a surge volume and will dampen pressure swings caused by the transient. It is the only place in the RCS that is at saturation conditions because it has heaters at the bottom. It keeps the rest of the RCS sub-cooled. For lack of a better term, it is like a spring and shock absorber for RCS pressure/inventory control. There is a spray valve that will open and spray water through the steam bubble to lower pressure if it gets too high during a transient.
During normal plant ops the Pressurizer level is controlled constant. The heaters cycle on and off to maintain pressure and there is a small amount of spray at a constant rate around the spray valve through a bypass into the steam space. The spray taps off the discharge of one of the RCPs and the bottom of the PZR taps into a hotleg. The makeup pumps ( charger pumps I think is the Westinghouse term) are used to inject water into the RCP seals at a constant rate and into the RCS through a level control valve based on the PZR level. The chemical volume tank is the supply for these Makeup Pumps. RCS letdown and RCP seal return are aligned to go back to the chemical volume tank. Between the 2 inputs and 2 outputs, there should be a near constant mass of water. There is a small amount lost through the last RCP seal stage and this needs to be made up. I can also divert the letdown to another tank and reduce my total inventory as needed.
Disclaimer: This is a basic overview for the B&W plant that I work at. I have never been to a Westinghouse or CE plant. I do not know how they are designed. I am assuming that it is similar.