It's probably the best of the duel skills because all non-seasonal daggers and around half the healers in the game are colourless infantry. However, I think I made an airtight case that as long as crowns aren't anything more than status and you don't care about arena ranking, it's just a bad use of orbs. (If climbing arena is a personal goal, then I think you should consider it, but keep in mind that healers have SP issues at the moment.) I'm reposting my argument; I don't think anything has changed.
https://fireemblem.gamepress.gg/q-a/cold-hard-economics-duel-skills
It's a valid point, though you could consider it to be economical if you pulled Loki on your free summon. It's also worth considering that healers have limited SP Issues, but a big aspect of that is the A-Slot which is occupied by C Duel Infantry. I agree with your points mostly overall but just some thoughts.
Colorless duel has the same conceptual issue as the other duels, which is that it only helps with arena score, which is inherently wrong on so many levels in terms of freedom of skill choice if you want to max your score. I won't recycle the points that you yourself mentioned, but it seems that duel skills are here to stay, which implies no arena changes, which is something that a large portion of the community (based on my observations) is not a big fan of.
I just hope that the nose-dive that the devs are taking when it comes to influencing the arena is not going plunge the game straight into the abyss.
The bonus unit kill points are exactly the problem and the duel skills only push that further. My problem is that duel skills imply that the bonus system is not going to change, because these two concepts go hand in hand.
It's like when you are grinding your character in an rpg to become a top level fighter so that you can go beat a boss with your party and when you get there, your teammate tells you that you have to run around the boss with your character in its underwear and waving a small stick to tease it, while your teammate solos the boss.
It's not much fun.
So just some more quick thoughts here (as another post since it's unrelated to my earlier comments).
It seems like the best recipients of C Duel Infantry will be Sothe, Jeorge, Loki, and Innes. These are the only colorless units with Prf weapons who don't strictly need their A-Slots to function well - units like Felicia have such low attack stat that without the A-Slot or a non-Aether Special can't deal enough damage. Sothe and Jeorge are both solid options that can be merged more, while Loki and Innes both have their Prf weapons but are harder to merge. There's plenty of other colorless units who'll do well, but just won't score as high because of the lack of a prf.
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