Full medic with quested gear, here. I think levels plays into the mix, as I was a few levels ahead even if I skipped Nar Shaddaa, but have since caught up. In general things hurts a bit more on Voss and Corellia. And Voss gave me the first two solo quests I could not complete alone (or was not patient enough to figure out). One is an instance boss many struggle with, and the other was either an area or bonus quest that in the last(?) step dropped two strong mobs on the top of me. The two strong mobs that could not be darted and disappeared on stealth, was way beyond what I could do with the current equipped companions (dps). Maybe a fully kitted Scorpio or Kaliyo could have snapped aggro fast enough for me to spam healing if I was really ready for what would appear. I will be back for revenge at a later time... In general story fights (either planet or class) that drops me in combat with multiple enemies have been the hardest. This is not so strange with a stealth class that can not cc while in combat.

I am a lazy companion healer on elites, and not being a pvp-er I am not so good at running around :-) When trying with companion, the avatar hit both me and the companion quite hard at once, so it may be just as well to use a dps companion while he/she lasts, and solo the rest. Or find another one that struggles at the same time. There should be a healthy supply of those.

I did this yesterday, and after figuring out he never dies, I started interrupting... No problem from that point on. I am healer spec, and he does not do much damage. I used Ensign Temple which should help quite a bit on the damage rate. I think he healed once or twice, the rest I managed to interrupt. And I agree on the avatar being hard. I found someone else struggling with the same, and with 2 persons he was quite doable...

You can see the level zones recommended for each planet on the galaxy map. I think Dromund Kaas is 10-16, so you are spot on. Obviously you get more experience before exiting the planet if you complete all heroic quests, pvp, do a few flashpoints, and play quite a few space missions. As long as you don't find solo content too difficult, your level if just fine. If too hard, try level a time or two on other content.

There is an option of what to show in the preferences. Start looking at the bottom of the social pane.

I saw prerequisite issues like these with the bonus series on Tatooine. That was about not having finished the main quest chain on the planet yet. Why you can get the bonus series before doing that is a different matter. I guess you have to advance your class quest. I skipped the planet before Tatooine, but did the class quest. I had no problems afterwards.

I have not payed enough attention to be sure, but I think I have seen this or a very similar issue. Some group member is low, I spam heals, and nothing happens to the health. The thing is that I am not quite sure if it is not some mob hitting at the same speed as my heals, but I figure it could be a recent stim boost creating a problem. If it is balanced damage, the spike from a 1k heal should still be quite visible. I guess I will have to learn to target back and forth real quick when triggering the boost.

This is indeed very annoying. I like using the operation frames in party. A nice little list of all members in one spot. But if a companion has to be used, I see no sign of the companion of another player. My own is at the usual spot down left, though. I discarded the party frames right away, once I realized my own frame was not in a list with the rest.