I think you were right on up until the bold part. When the IC joins the the unit he becomes part of it for all intents and purposes. Mordrak and the IC could still operate together since Mordrak was an upgrade Character to a unit that the IC joined. If the IC left, then I dont think he would be able to rejoin Mordrak.
I must disagree here, because this would be inconsequent. If Mordrak (or the Hive Tyrant) can be "accidentally" joined by an
IC due to the death of the unit they were attached to and the
IC had joined for a certain amount of time, then it must be possible to join them right in the first place.
If however it is impossible to join a unit only consisting of a single model, and if that is the case with a Hive Tyrant and Mordrak, then it must be impossible to join Mordrak with an
IC that joined the Ghost Knights when the Ghost Knights are dead.
My Argumentation is like this:
1. Mordrak is unit composition 1 (unique), so unjoinable by
ICs
2. Ghost Knights are 1-5, so joinable
The combination of both contains a joinable unit, so it is possible to join the Ghost Knights when Mordrak is with them. But when the Ghost Knights are gone, the joinable combination ceases to exist and Mordrak returns to a model in his own right. That means the
IC is no longer part of the unit since Mordrak has his original unjoinable status and not the upgrade character status of a ghost knight unit.
The Hive Tyrant is a little more difficult. The codex doesnt specifically state:
1. What is his status during his coexistence with the guard?
2. What happens when the guard dies?
Fact is, the unit Hive Tyrant is unit composition 1, so it is never more than 1 model. But the unit Hive Tyrant+Tyrant Guard is not so clear.
I would solve it like this (Attention, unusual...):
The Hive Tyrant joined the guard as if he were an
IC (he isn't though). That refers only to the joining process. That means from that on, he reverts to being an own unit but is locked in coherency with the hive guard since he is unable to leave the unit as he is only permitted to join, not to leave.
Other option: He becomes part of the unit tyrant guard. Then when the last "real" tyrant guard died, no
KP will be scored and the "Hive Tyrant" will be joinable after that because he is no longer Hive Tyrant but tyrant guard with a hive tyrant profile.
These are the two possibilities I can see. Both do not permit an
IC to join a Hive Tyrant per se.