So sorry to hear about the warped soundboard! That is a thing that can happen if the wood gets too moist and expands. Since it’s glued all the way around the edges, the soundboard has nowhere to expand except to bow up or down, and that’s when you can get a bump like you’re seeing.
Unfortunately, there’s not usually a lot that can be done to straighten it again, as the wood takes a set in that curved position. Drying it out might flatten it a tiny bit, and would certainly help to keep it from getting worse, but chances are it will never be perfectly flat again. Here’s one product that is meant to help regulate humidity: https://www.bovedainc.com/store/music-wood-instruments/
The good news is that often a warped soundboard does not affect the functionality of the instrument. If you can still tune it and play it, and if you can keep it dry enough that the warping doesn’t get worse, then there may be nothing really to worry about aside from the visual appearance. Are you noticing any issues with playability?
@DonO is correct that there aren’t braces directly underneath the bass bridges, and this is for tone reasons. The open bass sound comes from having a large expanse of soundboard that can freely vibrate. We’ve arrived at this bracing design after many years of building these instruments, and it’s a balance between making it structurally sound and also making it a beautiful sounding instrument.
The soundboards are dried to a very specific moisture content before being glued on the instrument, and this is also a number we’ve arrived at over many years of keeping detailed records. It means that dulcimers kept between 40% and 60% relative humidity should not have any problems with cracking or warping, but once you get outside that range, it becomes risky. It sounds like you’ve read a lot about it, but in case you didn’t see our super-detailed blog post (which talks about both dryness and humidity), here’s a link: http://manufacturing.dustystrings.com/blog/how-keep-your-harp-or-hammered-dulcimer-safe-cracking-winter-months
I hope that keeping it a little more dry will help to stabilize things! If you find that you’re having functional issues (difficulty tuning, buzzing, etc.) you’re welcome to email us directly and we’ll see if we know any ways to help!