Hi Phil,
Your piano is 14 years old. Has it ever held a tuning for a reasonable amount of time? If so, then it is not the pinblock-to-plate flange fit. That pinblock has been like that since your piano was new. There is about 20 tons of tension pulling on the 220 (plus or minus) tuning pins in the plate. Wherever the pinblock was going to get pulled, it is there and not moving. You have more than 20 very large wood screws going through the plate into the pinblock. If these screws are tight, along with all of the tuning pins going through wooden bushings in the plate and then into the pinblock, your pinblock is not "moving".
Pianos can go out of tune for a variety of reasons. If plate screws are tight in the pinblock and around the rim of the piano, the tuning pins are tight and the wood structure of the piano is OK, then it probably is humidity related. This doesn't mean the location of the piano is too dry or too wet, it means that the relative humidity in the room is changing. Humidity control systems assist in maintaining a more level degree of relative humidity in the piano. To check and monitor your relative humidity around the piano, purchase an accurate hygrometer. If your relative humidity is drifting more than 5 - 10 %, then the changes in humidity are affecting the tuning stability of the piano.
Tuning is a funny thing. Even the very best concert grands start going "out-of-tune" as soon as you quit tuning them. A concert grand is tuned normally before each concert. "In-tune" is really a matter of degree.
You said that Geoff tuned your piano and two months later installed some electronic parts and touched up some of the unisons. This sounds perfectly normal. Unisons going out of tune in 4 days, certainly possible, but then it is a matter of degree.
Should you have the pinblock shimmed? Well, it wouldn't hurt anything, other than your pocketbook. I have no idea, however, why it will cost $2,000. I see no reason why he couldn't shim the pinblock-to-plate flange gap in your home without lowering the tension of the strings. It sounds like the shims won't be much more than a business card thick.
Bill