I don't believe there is any material inside a 6821 that will leak. A catastrophic failure of the chip usually results in part of it burning or in some cases part of the plastic casing blowing off. Are you sure you haven't spilled something on it?

Do you have a logic probe? - it will make testing fairly straightforward. If you have replaced Q5 and Q6 as well as coil and coil diode then I would start with pin 4 of IC8. Remove pop bumper solenoid fuse, switch the machine on and start a game.
If everything is OK you should have the following results with a logic probe:-
IC8 pin 4 low, pin 5 high, pin 6 low
IC6 pin 11 high, pin 12 high, pin 13 high

Report your results. If you don't have a logic probe let me know and I will test with a DMM.

Andy