Hi Inflammo,
Dont know anything at all about folk music, but I would guess there arent many folk players who use a big 4 voice box for the job.
If you dont like it I would be inclined to try and sell it as it is, with the wet tuning, rather than spend a lot of money trying to fix a box that doesnt seem right for your purpose. A wet tuned box is probably going to be easier to shift than a dry tuned one if the retune still doesnt sort the issue out.
Even if you could find a repairer/tuner in you area I would doubt whether you would get a retune for less than £500, and you could add to that depending on whether you needed valves or anything else replaced at the same time.
If you like the Jonelli a retune may be a viable option, but youd need to be able to tell the tuner exactly how much wetness you need taken off, and that could be difficult if youre not entirely sure. Also bear in mind that a Pigini tuned to, say 14 cents, will probably not sound the same as your Jonelli tuned to the same degree.
The music of Colombia will probably be uncharted territory for you, and they tend to favour little diatonic two voice Hohner Corona accordions with various degrees of wet tuning. As such, I just cant listen to most of the music at all, especially when the accordion is being used to provide backing for a vocalist. However, every now and then youll get a player who plays an instrument and in a style that youre not particularly keen on, but who is able to make the most out of not very much. We can all be conditioned into what we habitually hear on You Tube, and hopefully this clip will demonstrate that.
My point is try and persevere with your Jonelli until youre absolutely certain its not the right one for you, or alternatively take a leaf out of this players book and just play the hell out of it!