i really feel like this depends on the champion. did you have specific champs in mind or was this more of a general thought experiment?

A general thought, but looking at Myciliac Priest Orn. On Hell Hades in particular Orn has this for traditional: https://hellhades.com/champions/myciliac-priest-orn/

and he also has this for Unkillable: https://hellhades.com/champions/myciliac-priest-orn/

For me running a Block Debuff comp, I'm wondering if I should be caring much about the defense tree - especially with champs that want to land / extend debuffs

So, if you're running a block debuffs comp without block damage, you would want the damage mitigation masteries at least on a few of your champions, in the Defense tree. If you're running both block debuffs/block damage, then we can look at Support for sure. 

So, if we're running say a Myth Heir with Heir cleansing the stun, that means we'll get value from Deterrence. If we're running a classic Maneater/PK comp, then we'll get value from Retribution.

Currently I'm running x2 increase defense champs with x2 decrease attack (Hotatsu and Sepulcher). 

Sepulcher's non-buffed defense is 4,658, while Hotatsu's is 4,385. Their buffed defenses are around 7,400 and 6,900 respectively. Mitigations are both at 84% while buffs active. 

Is the damage reduction from Delay Death additive? 

To your question: I believe so, yes.

In general I think I'd want offense/defense tree on pretty much all champions in these types of comps, racking up damage + damage mitigation is going to be pretty key.