I've posted this before but if you haven't replaced the rubber gaskets or injected something into the separations at the mitres, try this. Take some butyl putty (mine was just trimmings from rebedding a fridge
vent), Stuff the stuff right in the gaps at the mitres. You may want to take a q-tip and some Windex and clean those gaps of dust and grit to assure some adhesion before "stuffing". Keep pressing until it more or less conforms to the shape of the gaskets. Take a black MagicMarker and "paint" it to match the gasket. I think you have already cleaned the freeing ports in the sliding window channel. In a heavy rain, sheets of water on the glass will not be able to run thru those separations wholesale and the freeing ports will be able to keep up. I know this isn't very elegant but sometimes interim solutions can function for a very long interim. I think you will find that the replacement gasket will, in time, shrink just as much as the one replaced so this fix is worth some thought.