I wish I had such a nice dilemma to contend with
Seriously though, this is tough but as you say the economics of the situation sort of dictate. However you will still be able to enjoy MF photography with your film gear when you feel like it, I could see myself someday using MF film so its great you will still have this.
So, were I in your position I would sell. I've had the K-1 for about 6 weeks now and although nothing is perfect...it is my ideal dslr. I love how FF lenses look with this body both in terms of the dynamic range rendering and finally being able to utilize the native FOV. I'm sure you've experienced this and more already with your K-1. For me it was easy since APSc to FF is a step up in terms of the things I like to photograph (landscapes, closeups, macro primarily) plus I already had quite a few FF lenses so transitioning wasn't too hectic. I did keep my pair of K-01s (regular and IR converted) and a some DA limited lenses to use for a high quality very portable kit. I know many people have a dislike of the K-01 but it works exceptional for me for what I use if for.
In terms of UWA lenses, I've also been giving this some thought. The 15-30mm is nice but anytime I've had UWA zooms in the past...at least 95% of the time the photos I took with it were at the widest end. So in that respect I'm most interested in the Samyang (or some variant) 14mm/2.8. The cost of entry is very low for a lens that has the potential to produce as high an image quality as it does...yeah it has some quirks but I can live with that and manual focus doesn't bother me for an UWA anyways. I recently picked up an FA*24mm and it is stellar and wide already so the Sammy will definitely fill the UWA shoes for me while saving me a good deal of $ to use for another lens or whatever.
Your photos are great and will be great no matter the sensor anyways and you will probably be able to print large enough for your needs with FF. Good luck! Whatever you decide I look forward to viewing your work on whatever system you are using.