Gunpowder seems to be the kicker at the moment so I will join in a little bit.
I will take this point first:
6.5 years of research on a way to clear away material, tunnelling through rock, scaring away the dangerous beasts, etc.
From what is written the above, this is what I assume, that they spent that time trying to develop a way to do those things and almost solely concentrated on that.
I think you should stick with it being found on accident. Perhaps a small culture, like make China a village in your world, has been using the powder for years to scare away spirits and animals. Somebody discovers the town and their spirit scarer and realizes what it could be used for. It can be introduced to the 15 and they can perfect it.
I really don't like the idea of actually *trying* to develop something like gunpowder. Not that credible anyway that they would spend 6 years together mixing stuff without discovering a crapload of other stuff as well, plus they would probably want to work independently anyway I would think, traveling, exploring, looking for new dirt and flora to mix.... Why would a bunch of alchemists be thinking they could create something that could move mountains?
I just don't find myself able to swallow that a group would think they could solve the mentioned problems by mixing a couple things together to make it explode or to make something that could create tunnels? Surely they wouldn't think mixing stuff would create something like that...especially if there is magic in the world, that would be the first step. And if the kingdoms depended on making tunnels or passes then there would be tons of mechanical inventions for that purpose, magical creations, new boats to go farther, and simply physical labor creating some king of road over the mountains somehow. Six years is enough time to get quite a bit accomplished, maybe not finished, but at least a start of a few different attempts.
Some interesting points I found about gunpowder that could be neat and useful in a gunpowder setting...
Early makers simply pounded quantities of the three ingredients into powder and mixed them according to their own particular recipes. No finite method of proving the product existed; quality was judged by the loudness of the bang it made! Such a method of 'proof' seems funny to us today, but it was not quite so funny 500 years ago; 14th century Gunners were well aware of the effect of the sound of guns on an uninitiated enemy. They frightened not only the horses but also the ignorant and superstitious soldiery who saw guns as instruments of the devil - and Gunners as his henchmen! http://riv.co.nz/rnza/hist/gun/gunpdr.htm
Known as 'serpentine' (in allusion to an early type of ordnance) or 'meal' early gunpowder possessed several faults; firstly, jolting during transport caused the ingredients to separate, the heaviest ending up on the bottom of the barrel, the lightest at the top. The ingredients were therefore often carried separately and mixed on the gun position, creating in the process a highly explosive dust easily ignited by spark or friction.
Serpentine also absorbed moisture from the air to a degree which greatly reduced its efficiency, or in bad cases rendered it useless.
This is a good quick history too http://www.pyrouniverse.com/history.htm
I like that one and where I got spirits above.
An aside---don't call it gunpowder in game.
And if they were secluded....how did they become a kingdom? Perhaps instead of secluded you may want to define their needs as new trade routes to other impossible to get to lands. And movement by boat was much more economical and easier so they have to be inland lands they want to trade with or they themselves are somehow trapped inland...which will need a whole new story to support that as well.