I don't think it's very much like rape.
Yes, it is too large of a leap. Not all damaging things are rape. In fact, the vast majority absolutely aren't.
"Rape" is a word, much like "retarded," has a somewhat broad definition but has come to be used almost exclusively to mean one thing in common usage. Given that every single instance of inciting emotions/invading a mind in the books has been explicitly defined as a personal violation, violent, and destructive to the victim's psyche (on an often permanent basis, though you sometimes can learn to cope with such changes) makes "mindrape" an accurate word to me, insofar as I interpret how worming your way into someone's mind works in the books. (The main exception to this that I can think of is Thomas' work at the hair salon, but he's still invading their psyche and stealing their life-force, just a tiny bit at a time to minimize his impact as much as he can. The argument "it makes them feel good" when it comes to inciting lust has been compared solidly to arguing in favor of giving drugs to a drug addict in my eyes and I won't rehash it.) If your position is really that using rape in a different way that the common usage somehow diminishes the seriousness of rape or the damage it does to victims, I'd argue you're wrong, but this is not the forum for that discussion. Since I'm not out to offend people with my word choice:
Music incites emotion. Naturally. It's pretty innate in humanity so I don't see how having incite emotion would be that terrible. It's just a mechanical way of representing a very skillful, supernatural music.
It all depends on how you narrate take outs.
I'd say the Devil's Gifts are Tainted and using full-on Incite Emotion is still an invasion of someone's mind and a Bad Thing.
In the books, there's isn't a single instance that I can recall where such a thing wasn't a personal violation, violent, and destructive to the victim's psyche (on an often permanent basis, though you sometimes can learn to cope with such changes). This is certainly the case with and point of Lasting Incite Emotion, at the very least. The main exception to this that I can think of is Thomas' work at the hair salon, but he's still invading their mind and stealing their lifeforce, just a tiny bit at a time to minimize his impact. The argument "it makes them feel good" when it comes to inciting lust has been compared solidly to arguing in favor of giving drugs to a drug addict in my eyes and I won't rehash it.
Since the book (which I understand is not canon) says "Dark or morally “corrupt” emotions (lust, wrath, despair, and others) are the usual ones available," and this is a fiddle
made by the Devil himself, using its power to
force an emotion from someone is inherently tainted and a Bad Thing. Music can elicit an emotional response from people--but not every piece of music does this for everyone, the emotions people feel when they listen to the same song can from person to person, and the emotional response you get from listening to music isn't based on whether you're incredibly Disciplined or not. Furthermore, the intensity of the emotions they feel isn't determined by the musician (via the value of the roll in our game) but their own personal psychological response to the music they're hearing. The musician's skill obviously does affect this, but it does not define it.
Unless there's a legend about this outside of the Charlie Daniel's Band song that I don't know about: Johnny wasn't looking to get better at the fiddle, he boasted he was the best fiddler ever. This drew the Devil, as such boasts are wont to do, who bet Johnny he
wasn't the best, and enticed him with a fiddle of pure gold (which, by the way, would weigh at least 18 kg/40 lbs using standard violin construction, and sound terrible...but this is magic
). Unlike something like the Crossroads stories, this one isn't a trade of soul for skill, it's just an instrument the Devil played...and the Devil gifting a cursed Fiddle that grants power at the expense of that power hurting those around you whether you want it to or not sounds right up his alley.
That's just where my thoughts are with this item. The truth doesn't have to follow the legend completely, and you're welcome to interpret Incite Emotion differently and/or allow emotions that aren't "dark and morally corrupt" if you want to. You could build a hundred different variant fiddles and not break the surface of possibility.