For people to take control of your title they need a casus belli (if you have an heir then you'll most of the time be somewhat secured) but yes, you will most likely be able to hire npc and pc guards.
Yes, though you can't assign positions as an aristocrat as that's a "lower" title in the hierarchy, though every aristocrat resides over a settlement (barons are also mayors) and they oversee the court of commoners. Though the King's court is somewhat different, but you will most likely need to be king or duke to appoint people to those kind of courts, or have earned the privilege. though the court of commoners will be judged by the Town Council (land owners most likely) and some specific people who we don't know how they're appointed now. (Though a lot is still subject to change)
No, the game is still in early development, though the ElyriaMUD is soon-ish (take that with a grain of salt) to come out. I would suggest checking out the Developer journal on this one which can be found by clicking on this highlighted text, about the pre-alpha experience. (And if you don't want to read all of it then scroll down to ElyriaMUD).
A Baron is someone who made a contract with the duke, he will then receive extra payment to uphold a garrison, so it's a way for the duke to "secure" some choke-points or just borders where fighting might erupt, come wartime. It's important to note that the Baron is still a mayor, and so he still pays the count of the county.
For some it is, for others it isn't depends on what kind of playstyle you're looking for. Though there's a difference between nobility (kings, dukes and counts) and aristocracy (barons and mayors) but it's also worth remembering that every king is also a duke, count and mayor, and as every count is also a mayor etc. so you won't loose a title that's under you if you move up, though you will take a much bigger spirit hit the further you go up the hierarchy, if someone were to kill you.
Though it's also worth remembering that the world will start with 6 kingdoms, 12 duchies pr. kingdom and 24 counties pr. duchy, that's 1728 counties on the world (though a lot of them might be merged with existing counties as the kingdoms of elyria starts playing out, but there can also be more than 1 settlement pr. county, so it's not like there's only 2 duchies in the world and that you'll always be the target of some crazed assassin plot :P
