Salt is definitely the single most important seasoning component in meat cookery, IMO.
Let me see if I can help out a bit with your issues. Regarding the 'saltiness' of your rub using your 'usual' coarse sea salt amounts. Are you creating your rub recipe by weight or by measure (i.e. cups/tablespoons/teaspoons)? Depending on the company of origin, the actual weight/amount of salt could differ substantially depending on the grind/grain size of the particular salt used. Weight is ALWAYS the best way to create consistency in your rub blend (as well as using the same source provider for salts, peppers, spices, herbs). I'll also throw out there that I once, in a pinch, bought some coarse sea salt that was Portuguese in origin that, for whatever reason, had a very sharp saline bite to it compared to local (Windsor salt) product of the same size grind. The 'flavour' of salt essentially comes from the residual trace minerals, so it the origin and size of crystal that will most greatly affect the 'flavour' of a particular type of salt.
Salt in a rub is a key component in the balance of sweet, salty, savory and heat. You can do a salt-free rub, and it can be tasty, but it may not be that nice balance many seek. On the other hand, if you are brining meat beforehand, a salt-free rub may be beneficial/necessary to compensate for the residual saltiness in the meat from the brining process.
For all intents and purposes, coarse sea salt and coarse kosher salt are inter-changeable, but again, the crystal size and trace mineral 'flavours' can vary in each designation, dependent on supplier. My strongest recommendation is to find a source of kosher/sea salt and stick with it if you are consistent results in your rub blend. Oh yeah, and do weigh your ingredients versus using volume measures......
Homefires Burnin' BBQ Team