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Chaos Space Marines with Allied Chaos Daemons. Rules that matter;

BRB, pg 112 under "Battle Brothers and joining units"
Codex CD pg 26, Daemonic Alignment
Codex CD pg 26, Daemonic Instability

Can a Daemon Herald join a unit of CSM that have the Daemon rule? (Possessed, Obliterators, Warp Talons, Mutilators).

a) Battle Brothers, so IC's can join Allied Units and their powers will affect allied units as they are friendly
b) Daemonic Alignment states that they can only join units comprised of Daemons of the same alignment
c) Daemons from Codex CD have the Daemon USR.
d) Obliterators, Possessed, Warp Talons, Mutilators all have the Daemon USR, and can also purchase marks of chaos
e) Daemonic Instability states that a unit with this special rule cannot be joined by a character without the special rule
f) Daemonic Instability *does not* state that a unit without this special rule cannot be joined by a character with the special rule

So a Herald of Nurgle is not restricted from joining a unit of Obliterators with the Mark of Nurgle (similar alignment)? Is a Daemon of Nurgle the same as a Daemon with the Mark of Nurgle in terms of alignment?

Thoughts?

This leads to a very tricky scenario, because what happens if a unit suddenly becomes Fearless (say with Telepathy); since they can then no longer lose combat, is the Herald now immune to taking a Daemonic Instability test?

It's unclear whether the wording is referring to "Daemons of" certain gods, and thus only applies to models with a rule that starts with "Daemon of" or if it refers to "Daemons" of certain gods, and any model with the Daemon rule qualifies.

As it only mentions "Daemons of the same alignment", and not the defined terms "Daemon of Slaanesh" (etc) currently you CAN join. No idea if this is intended

Yes, this one is going to need an FAQ. Your logic is pretty solid -- it's just that it's vague whether the generic Daemon rule and the mark of the appropriate god is equivalent to "Daemons of the same alignment." Given that Epidemius was nerfed so as to only effect Daemons of Nurgle, it is probable that the intent was to keep any ICs from the Daemon Codex from joining anyone not from the Daemons Codex, but that GW just worded it badly.

A mark of Nurgle Obliterator does have the same alignment as a Daemon of Nurgle - they both worship the same God. That is the only possible, in context, meaning of "Alignment"

I expect them to change it, as apparently it just isnt cool to mix morttals and immortals together (sniff, no jugger lord leading a unit of juggernaughts, despite that seeming a really blindingly obvious match up given the lack of cavalry in the CSM book)

By definition, a Chaos Daemon IC is a unit on its own, right? So by the Daemonic Instability rule, is it able to join another unit without DI at all?

In other words, doesn't argument E invalidate argument F?

No.

A unit WITH the rule CANNOT be joined BY a unit WITHOUT the rule

There is NO requirement the other way stating that a unit WITH the rule cannot JOIN a unit without the rule. The only reuqirement is that they are daemons of the same alignment.