SS/FG bikes are less maintenance, in that there are no derailleurs to go out of alignment & fewer cables to deal with. Plus, they are fun. However, because you can't shift, hills/wind/starting from a standstill are all less fun. You definitely get more versatility from a bike with gears.

Drops will give you a more agressive posture - ie more aerodynamic. I really like drops, but that is a personal preference.

There is no reason that one bike can't fill all your needs. I like bikes - and have more than one because I can justify it. I rode my road bike to work for several years, it worked fine, although with the very narrow tires it is a little more jarring on anything not perfectly smooth (hence my desire for a "commuting" bike).

I'll leave the other questions to someone else - gotta get to work.