Regarding the sealant question, BoatUS latest issue just had an article about sealants/adhesives for bedding deck fittings. I found most comments applicable for our needs as well.

They recommended against silicon as it tends to weather quickly and give up, and leaves a residue that nothing will want to adhere to.

They also mentioned that the 3M 5200/4200 stuff should be used very carefully and only appropriate for 100% permanent needs.

The stuff they recommended is Polyether based 3M 4000UV or West Marine Multicaulk. Seems to hit the sweet spot for flexibility, bond strength, weathering, etc.