No matter how high my Diplomatic Relations get with, e.g. Tikanan, their ships always attack mine on sight, so I can't entwine the system.  Does this mean first I have to destroy their fleets, and only then entwine the system and assimilate them?  That seems... unsatisfying, that the Unfallen have to wage war where any other faction can just click the "Assist" button and get their quest, particularly given the pacifistic bent the Unfallen are supposed to have, lore-wise.  Am I missing some other way to do this?

Are you the sovereign of the minor civilization?  I think that even if your diplomatic relationship with them is 100, but another major empire beat you to sovereignty, then the minor civilization is allied to that enemy major empire.  If you are listed as the sovereign and they still attack you, then you should probably file a bug report with a save game attached.

-HP

Oh, so Minor Civilizations aren't supposed to attack if you're their sovereign?  OK, sounds like a potential bug then.  Thanks!

Unfortunately, my Unfallen playthrough was before the patch, so I can't say for certain.  I was just hypothesizing that perhaps an enemy empire being sovereign was the cause.  Maybe the devs intend for warmonger minor civilizations to even bite the hand of their sovereign, but then I'd agree that this makes it really hard for the Unfallen since they have to entwine before assimilating while every other empire can do it remotely.  I'd still file the bug report.

-HP