Yes, it is possible for an overfilled tranny/clutch case to cause oil to be puked out the seals. A little over won't do it, but way too much will.
Damage to the seal is another possibility, and normally the cause, but if both your shifter and countershaft seal are gone, it is somewhat suspicious, they normally don't blow both at once without other issues. The cases leaking and pressurizing the tranny/clutch case is one possibility here.
The shifter and countershaft seals can both be replaced with a seal pick to pull them out, and then drive a new one in with a socket or seal driver, being careful to clean things well and not damage the new seal. The only things that need to be removed are the sprocket and shifter, to gain access to the seals.