1) The instrument panel is electronic, save for the speedo needle. As far as I know, these panels tend to malfunction after some tears of use. My dad's 325e has the fuel consumption gauge and which sometimes acts erratically along with the temperature gauge (located just above the economy gauge). I'm aware that the problem can be solved by removing the panel and replacing some rechargeable batteries, which also cures the service interval indicator and oilservice problem (they refuse to stay off even after a reset has been made, and the batteries are there to backup the state after the reset- if they are bad the computer never resets properly) . The funny thing is that almost always the service lights problem appears first and then the other gauge problems.
I believe the complete solution was in BMWe30.net, just in case (post here if you want me to put it here, I think I have it in one of my older backups)
2.- I don't know if the economy meter is connected to some vacuum line that senses the vacuum in the throttle body, as it is on some older or simpler cars. I think the economy meter is connected to a signal from the DME, since the DME always know how much fuel is injecting (via the airflow sensor). I don't think the germans would put a vacuum line to the throttle body if they already had a means to electronically measure it. But on second thought the E30 was designed in the early 80's and many models were carburetted (the 315, 316 and 318), so maybe the vaccum system is there.
3.- If you have a vacuum leak in the intake tract that would affect the performance of the brakes and also the readings of the airflow sensor, which in turn would also affect the gauge reading. You should also be experiencing severe problems with drivabitlity and idle due to the varying vacuum (altough I wouldn't drive a car with a brake pedal going to the floor). So yes, the two problems may be related, though I don't see how removing a little pin would have an effect in your brake system (especially if you assembled it all back together) Maybe you messed the electronics?
Hope this helps