Four have always drug me back to them. Putting up spoiler here just because I'm going to actually delve into a couple of these a bit and want to be on the safe side.

I'd love this. Get some relevance back to our ships and make it feel a lot less lonely.

Zakuul and Korriban.

Ffon from the SI prologue, would've happily taken him as a slave over Khem.

Veeroa and Senya are currently my two favorites with HK-55 bringing up the rear.

Cytharat would be one I would love to see again.

If I remember correctly, they stated that companion quests would now advance with the story, rather than being based off influence.

Datacrons have become legacy bound I believe (at least the ones without the shards) so it sounds like you are trying to re use a datacron that you have already unlocked for your legacy.

I imagine that the home planets are set up similar to how the starter planets were (meaning no actual storyline but a bunch of missions in each zone) so you may find some purple ones for the planet as you progress.

the 12xp will only last until level 55, at that point it is meant to revert to the normal level of xp gain.

Looks awesome and would love to see my warrior decked out in it, so definitely supported.

I'd say HK works well with sith characters as well as agents. Depending on what kind of smuggler or bounty hunter your using, he can work well with them as well, though I would still say sith and agents are the better option. Treek makes, arguably, the most sense with bounty hunters and smugglers. However, due to her merc nature, most can easily work well with her. The only exception I'd make would be in the case of trooper, though even that can be worked around.

You can start up the expansion quests (I assume you already have the expansions) through a mission terminal on your ship. As for PVP, I might recommend just playing a couple random matches to get a feel of how your toon plays now.

Blood Hunt is a part of the SoR story, continuing along the Rishi quests will eventually get you there. You will later come along another flashpoint outside of Rishi Village (I believe that's the place) that one will also gain a mission from the main story line before entering.

Upon reaching max level, you would typically do daily areas (you get leads to some of the worlds that would follow under this category), pvp, or Flashpoints/Ops (using the WoW example, these would stand in for dungeons/raids respectively). There is also the option to RP, if that is something you are interested in getting into of course. Without the current expansions, you might find a lack of interest in some of the older flashpoints and operations, but it shouldn't be impossible to find some interest for them.