To replace the water pump, the timing belt has to be removed. If the shop you used was any good, they would have at least advised you to replace the timing belt and balance shaft belt at this time. A sudden drop in gas mileage raises the possibility that the timing belt was not reinstalled correctly.
Worse case, if they did not replace the belt and they reinstalled the original belt to rotate in the other direction, then that belt could fail prematurely.
The valve cover gasket is reuseable if not too old, but whether new or reused, there are two small areas that have to have a sealant applied every time the valve cover is removed, otherwise it will leak. I suspect though that the guy from the parts store noticed oil pooling around the base of the spark plugs. Since the spark plugs are in holes in the valve cover, there is a seal for each one of them.
New seals come with a new valve cover gasket set. That makes me suspect that the mechanics who did the job on your Honda threw away the seals instead of replacing them and they don’t want to buy a new valve cover gasket set just for the seals, its not cheap.
If you have a Honda dealer nearby, I strongly suggest that you get an estimate for a timing belt job from them. I have a 97 Accord, which is the same generation as the 92 I believe and the timing belt job, which included a new balance shaft belt, a new water pump, all oil seals on the front of the engine, new valve cover seals, fresh coolant and a new oil pump seal ran me $735.
If you have more than one dealer, check them all. This was the second timing belt service I had done at a dealer. The first one, 8 years ago at a different dealer actually cost me more and I don’t feel they did as good of a job, I don’t think they got the balance shafts timed properly, which on this engine is really tricky if you don’t have the actual factory shop manual to show you how.
But the guy who did it this time got it right and the engine is much smoother now.