A blocked tank vent wouldn't have any impact on a pump that's beyond a closed y-valve.
A pump can only leak if the seal or gasket that seals the leakage point in the pump has failed. Which means it's actually continously leaking, but doesn't appear to be when in use 'cuz waste is going through it. But if you check it several hours after use. I'm betting you'll find that it's leaking, no matter which way the y-valve is aimed (if aimed at the macerator pump, you can't use the toilet during this test).
When, if ever, have you rinsed out the macerator pump and plumbing to it? That should be done at least nominally after every use...but at LEAST several times a season.
-Peggie