The 7A24 seems attractive because it is 400MHz bandwidth and dual channel. You may find it less useful than you expect because the inputs are 50 Ohm impedance and you will not be able to use the commonly available probes. You can make your own probes for 50 Ohm inputs but you typically get a 500 Ohm or 1kOhm impedance at the tip and they are rather a specialist item. The 7A26 is more generally useful and there are a lot around. eBay prices seem to be forever on the increase, check for completed items to see what they actually fetch.
There is definitely something wrong around the +/- 9V regulating circuitry. There should be -9V on the Zener diode VR550, you could try measuring that and also the voltages on pins 6, 7 and 10 of U550. I am surprised that the Invert position is still showing the same Y (lack of) sensitivity as before since adjusting the +9V regulator should also have changed the -9V supply. Fixing any problems that you identify is going to have to wait until you can get the 7A12 out of the frame.
You can find replacement plastic guide rails should you break the one currently holding the 7A12 in place. Qservice in Rhodes (Greece) is a well respected supplier and has part number 351-0181 advertised on his eBay site. If you were to ask around on the forum you might well find someone who has parted out a 7000 frame and would sell you one very cheaply (I don't have any unfortunately).