You've really got a couple of questions here: one about string lubricants, and one about whether calluses eventually get hard enough that it's not an issue.
I've been a pro guitarist since 1976 (and yes, it's my full-time job). I never use string lubricants, and I'm not aware of any other pros who do.
Fast Fret is marketed as a string cleaner, but I think it's just mineral oil - I couldn't find an MSDS online. Any oil will prevent dead skin cells from sticking to the strings, which will prolong their life - if you're interested in why, I can go into more detail. But if you're a working guitarist, you're going into gigs with new (or at least new-ish) strings, so intonation isn't going to be an issue.
The oil will repel water, so if you sweat a lot I guess that might help prevent strings from rusting. But I just keep some talcum powder in my case in the event I have a gig in high temperatures, so that's never been an issue for me.
FWIW, a competing product (Finger-Ease) is mostly naptha, a solvent. I do use naptha to clean my fretboards when I do string changes, but I use the much cheaper version - old school lighter fluid. It's the same stuff without the aerosol propellants. But I don't use it on the strings.
For the second part (the calluses), the answer is yes/maybe. When I was younger my fingertips looked like glass - perfectly smooth, you couldn't see any fingerprints, and if I tapped them against a piece of glass they sounded like metal.
They're not like that anymore, even though I play just as much. The difference? I have better gear, and it's better maintained. When the best instruments I could afford weren't really that good, thicker calluses were the result. As I was able to afford better gear, my calluses downsized. Now they feel kind of like smooth leather - there's a bit of 'give' to them. I can still get a clear sound by tapping on a piece of glass (I just tried it!), but it's not the clear metallic sound they once had.