Just wanted to start a post about the same experience. Fitting new front screen rubber, I have also a gap between rubber and chrome panels at the top edges (with screen supports in the top position), so it´s more or less ok.
I think, it´s necessary to use a proper mastic for screen sealing in order to hold rust away from the A-post. For sure, not at the chrome parts, but between new seal and body. On my car, there was no bottom support channel installed and so no rubber with the channel. Instead of this, a huge amount of some mastic, which was not proper for screens. It took me 3-4 days to cut off all this stuff. Due to the missing bottom channel rubber, water always came down to the bulkhead and finally into the passenger floor, I think, at the heater box. Seems like the bulkhead insulation holds water also. Next days I will take out the heater and see what rust level looks to me :roll: