How to paint over unpaintable silicone

You can fill the gap with sealant, then let it set (which might take longer than you think, even a day or two) before doing the actual seal.

For the one where the sealant is failing, there are two main causes - either it didn't stick very well in the first place or the two sides of the seal are moving apart (hence the advice to fill a bath before sealing round it). If there's movement you need to deal with the underlying cause.

If its not sticking very well, cleaning the surfaces with methylated spirit (denatured alcohol) may help.

Every time I used any type of grease on a rubber type gasket the gasket would slip out as the screws/bolts were tightened. My recommendation would be to use RTV silicone instead of any grease. The RTV can be purchased at an auto parts store or at most home/ hardware stores. It will seal out any water, dust, etc, better than grease.