Without getting into board level troubleshooting, it sounds like it could be logic board issue. The deteriorating trackpad issue could have been a slowly dying component that was drawing too much current and overheating. Eventually this could damage whatever is driving that current from the logic board side. Replacing the trackpad with no effect is further possible evidence. Is the macbook working normally otherwise? One typical issue is it will go into "limp" mode - the trackpad has sensors on it that the SMC uses. If the trackpad sensors are not working the SMC will force the CPU into a mode where it runs at a flat 0.8GHz and the fans run full speed. The idea is that SMC can effectively manage the mac so it forces into a mode that is ultra safe. If the macbook is working fine otherwise (not in limp mode) then it may be that the section on the logic board that interfaces with the trackpad resposible for the pointer function is not working (and that sensor connection and function is intact). You could try another trackpad or try the trackpad you have on another machine - you want a verified working trackpad. then if it doesnt work, it means either connecor and/or cable and/or logic board issue. This is the "fun" of troubleshooting this stuff. good luck!