Um, you mean Volc Demo suit? Vol demo helm has Ctr med, while the suit has Ctr low.

The other day I bought and learned 35,000 crowns worth of recipes, before realizing that the 5-star item I was going for didn't do what I wanted. So I was upset. But I was upset at myself, because after all it was my mistake. I hope that you're mad at yourself, not the game designers, and that you forgive yourself and have fun.

just hang on to it - it may seem bad now but someday you'll want it for a costume or something.

If it makes you feel any better, Medium Fiend isn't a game changer. If wearing only one piece of Divine it makes sense to get the veil instead since the mantle lacks the bonus, but with most encounters it will be completely unnoticed. ~15% boost on damage to fiends is rarely going to reduce the number of hits necessary to kill them... if any enemy took 7 hits to kill then you can finish them in 6.

I don't really pay much attention to which sets do this, but I got the impression that when the armor & helm stats differ it's the top piece which usually has the little advantage. If it actually is always the helm, it might be to restrict us to using only one such bonus at a time. I think the wiki pages for some items ought have flags to indicate mismatched stats for some armor/helm pairs, and that there should be a category for such sets, but did you seriously say you started working toward an item and are now upset because you found out that another item is slightly better? I fail to see getting upset as reasonable; if your enjoyment of that Shadow defense is totally ruined by finding out that you could have gone for something slightly better had you noticed it earlier, I think you need to grow up rather than pester the GMs or any one else working hard to bring us this game. Most of us make investments we kick ourselves for later, it's part of life and it's part of this kind of game. I hope you can appreciate and enjoy the fact that your cloak is still really good as gear choices go; damage resistance is competitive with other options for both of its types and status resistance is way above average in usefulness.

If you know for sure you will never have desire to stack both Divine pieces, either keep enjoying what still seems from your OP to be exactly what you expected it to be, or sell it and start over for the bonus against Fiends. Were I in your shoes, I'd just keep the thing even if it were still at 2-star. I myself am going for Divine/Skolver hybrid as my primary armor set and am still working toward 3-stars but already have the second Magic piece so I can pile on extra Fire and Shock resist if I have too much trouble somewhere. I also have the second Wolver piece since I'm working toward RJP, for that matter. Being able to double up and focus on more specialised defenses might come in handy if you end up having trouble with a particular stratum, and Divine with its Shadow & Elemental defense and excellent status resists is probably more likely than most armor sets to cover the specialty you need.

I myself would very much prefer that all options occupy the Pareto frontier and could go on for pages about why I do, but am satisfied with having a wide variety of options that are at least viable because I'm not compulsively bound by such concerns. (I like Troika's funky shape and charge attack sound effect, so I got it knowing full well that the sword Kozma sells would probably be more effective.) If any piece of equipment I like the mechanics of is effective enough that I can play the game the way I want, I should enjoy using it. I obviously cannot offer knowledge of why Three Rings did any of the weird things they did in SK, but can offer a possible motive that might be comfortingly elucidating. If there are any motives for a player to choose something because it's suboptimal, there is at least one motive for the developers to make such choices possible. Stotting is perhaps the most obvious one. Also, some players like to handicap themselves to keep the game challenging and interesting for longer. I've also been told that many people feel better about their choices if they can be seen as distinctly above-average in desirability. I don't have an opinion on whether that's actually common, but I do think that for too many people choices which are clearly irrelevant from a game theory perspective are still relevant on a base emotional level. I've also noticed that many like to do things because they are unpopular, and creating gear that's distinctly overshadowed by other gear but still plenty viable would be an obvious way to cater to them. It's a minor annoyance if I happen to like such an item, since it discourages me from playing in a party, but my schedule is too irregular to form a regular veteran party and I lack the skill or CE to handle PUGs too often anyway.