Even if Iku/Tenshi's damaging roles are kind of redundent, Iku is still an excellent buffer and Tenshi's buff removal is pretty useful in Plus Disk. Kogasa and Parsee can deal huge damage, but you've already got several people who deal huge PHY/DRK/CLD damage. Plus, unlike Iku/Tenshi, they don't really have any utility you don't already have covered.
Well, the fact is that Renko and Maribel already cover those roles pretty well on their own - Renko has one of the best buff spellcards in the game (Charge), and Maribel has one of the best anti-buff abilities in the game (Ability to See Boundaries). I've seen plenty of fights vs buffing bosses where my entire team died except Mari (who took, like, next to 0 damage from everything) and Renko (who has massive defenses + evasion and can tank just about everything the game can muster). Heck, there are times where Tenshi removing buffs is actually something I do NOT want to do (for example, vs Ame-no-Murakumo) because of Maribel.
You are right about the fact Kogasa/Parsee don't really bring anything other than damage to the table, but at this point, it becomes a matter of what I actually need in the first place. As far as elements go, I have pretty much every element covered except Fire, which in this game unfortunately doesn't have any really standalone strong nuke besides Flandre. And I have only 2 slots free I can really spare, since Reimu, Sanae, Miko + Futo, Renko + Maribel, Nitori and the oni family are kinda non-negotiable for me (at the very least, I can't think of better choices for boss battles in general, except for some minor niches).
One possibility could be of employing Kanako and Suwako to complete the Moriya family, but neither of the two are especially strong on their own without the kin boost, and with several lategame bosses having the "team scramble" ability, I'd prefer my synergies to be more versatile. For example, while Yuugi prefers being slotted with the other oni, her PHY damage boost synergizes with Futo as well. Similarly, Renko and Maribel (and, to a lesser extent, Reimu and Sanae) can support just about any two random characters. Both Nitori and Miko are fierce attackers on their own, as long as they're boosted, and the latter can debuff the opponent as well.
(Also, I fully get it that PD content is currently not all that difficult and I might be trying too hard for what it is, but what can I say? I like to experiment, and I'm an inveterate perfectionist)
Warrior on Kogasa is pretty good. Without it, she has exactly one good element: DRK. With Warrior, she becomes a good attacker for FIR and CLD as well. As for Murakumo's Blessing, there's really no bad choice. Just pick your favourite character and laugh as they destroy everything[Or laugh as they help everyone els destroy everything.
Well, there are some chars where it honestly would feel wasted (like, say, Reimu or Miko). As the description on site says, the best thing to do would be to give it to a char with naturally high stats (especially MAG) and a lack of good personal spellcards. Most of my current party members either are physical attackers (and have stronger attacks than Murakumo Slash) or have are magic attackers but have a SPI-based attack already (Sanae, Reimu, Miko) or better attacks to use (Maribel). Literally, the only realistic candidate I have as it is would be Iku, and she sounds far from the most exciting user of the subclass to me.
IMO, adding an Extra Attacker instead would be better to allow you to reduce delays, other than that I would say you have enough attackers as it is, so adding some extra support options with characters like Akyuu or Aya may be a good idea.
The only Extra Attacker I could consider as a standalone option is Yuuka Kazami (who, by the way, might also make some decent use of Murakumo's spellcards). But even with her high stats, she lacks %-based ability boosts of her own (such as kinship boosts, or stuff like Kogasa's TRR boost) besides Tormenting Nature, so I'm not sure her damage output would be as good as one can hope (it certainly wasn't in the main game, though that was largely because of Yuuka's poor defense piercing so I'm not 100% sure).
As for Aya, I've considered to use her - maybe alongside Momiji and Nitori to take advantage of their awakening. But that is basically incompatible with the original plan of an awakened Oni team, so I'd have to decide which of the two is better. In the main game, the Oni were definitely better (largely because Yuugi especially trounced nearly everything with Knockout in Three Steps), but maybe in the postgame Momiji and Aya could do more damage over time due to their immense speed and low delay attacks (combined with the fact defense piercing is not as crucial in the postgame as it is in the main game). Plus, while a Nitori with +30% physical damage (from Yuugi) is fun, a Nitori with +42% to all stats (courtesy of his awakening) could be really fun. But that brings up again the point of what happens when the team is scrambled by Second Sun and similar bosses (on their own, the Oni are much more powerful and useful than Aya, Momiji or Hina).