Man, I hate multiquote sooo much. I can barely do a single quote because I'm using ENworld via a phone and Chrome browser.
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I think that's all the points...
I appreciate you taking the time to go through all this, but there's simply too many variables. Now, if I had been there to view the combat, I might be able to offer a more thorough critique.
Anyway, I'll address a couple points and leave it at that:
Kobold? There's your poor choice right there!

(j/k)
But, as I said, 10th-level druid average 53 hit points. You took 52. How badly were you already injured? Were you not able to heal up before the BBEG fight?
Also, if there was any bad luck on the dice rolls it was the
sunburst damage roll. Average would be 42, rolling 52 or higher is about 5% of the time. The barbarian should have had a good chance to save, but I don't know the save DC but I would think it should have been about 50/50 or better.
Sunburst doesn't go around cover (or maybe just total cover if you had it?) like
fireball, for example. So, if you had total cover, it shouldn't have affected you IMO.
The zombie giant (I would imagine) would have been in the AoE as well, so that should have just about killed it after the
heat metal for a couple rounds.
Given the apparent strength of the opposition, I also don't think splitting up was wise (but hindsight is always 20/20, so...).
From the sounds of things: powerful caster (8th level spells minimum), zombie giant (CR 9 or so), and some guards. For 10th level PCs this sounds like it might be a deadly encounter anyway. In which case, even if you are making the best choices, the fight can turn against you.
Anyway, as I've said a few times now, 90% of PC death isn't
just bad rolls. Rarely it is only bad luck and poor dice rolls.
This is sort of the problem I have with @DND_Reborn drilling down into @Sabathius42 's story as well.
For the record, bad luck can happen, sure, but IME PC death is due primarily to the way people play their characters 90% of the time.
You pushed your luck when your hit points were low.
You failed to act when you should have.
You didn't understand how a feature worked properly and you (or another PC) died.
You weren't prepared (lack of equipment, knowledge of the battle field, etc.).
Yes, there is always that
chance that something happened entirely beyond your control, but it is pretty darn rare. There are also times when the above happen, but you feel you don't really have a choice. Whether or not you
actually do is debatable.
Finally, I am not "drilling down" into their story. Yes, I am questioning it because they brought it up as an example of "playing right" and just bad luck. If there was any "just bad luck" it was the damage for
sunburst. The damage the spell does is completely out of their control and had only about a 5% chance of rolling that high (or better). Now, maybe less damage would have finished of Yip-Yip. I don't know. But even 52 damage should not take down an average 10th-level druid, even without a CON bonus.