Control panel
Looks all original. Hard to tell the buttons are for sure due to video quality but they look and sound right from what I could tell. The plungers should be relatively tall, compared to every other button ever in an arcade cabinet. The stick definitely looks too stiff. Sticking down might be the spacer missing between that microswitch and the main body of the joystick. That was the case on my red dk when I got it. I ordered a replacement from mikes arcade and it works great. And of course I found the original a week later, under the power supply/iso mounting board. But it had corrosion starting to build on it so better it was replaced anyway. But the stick being stiff could also be someone replacing the stock spring with some junk off the shelf from home depot. Mike's arcade has good replacement springs also. But, first things first, just tear down the joystick, clean the crap out of all the various components, and throw a drop of 3-in-1 oil in the pivot ball area when reassembling. Mike's arcade also has a schematic of the joystick and tons of schematics, pinout, etc, in addition to tons of great replacement parts. Have i mentioned mikes arcade is awesome?
If after a good cleaning and lubing, it is still stiff, then diagnise what the siurce of the problem is, but its likely time to get a replacement spring.
Next question from your video - mikes arcade also has a Nintendo to jamma adapter. It plugs onto that edge connector you were noting. The "rainbow" cable that is connected from the upper left hand side of the video board to the upper right hand side of the cpu board is needed if using an adapter to run it in a jamma cabinet. That routes power between the boards. So good news that you have that cable. HOWEVER, donkey is displayed rotated. Monitors in dk cabs are actually rotated. So if you try to hook up that board with the nin2jamma adapter, in your sf2 cab, it will be sideways....unplayable. if the sf2 cab is the only cab you own, then youre out of luck with that option. If you own a vertical monitor cab like sum up then it will work and be displayed correctly.
Also, just for extra information, that rainbow cable will not be needed in an original dk. Do cabs all wire directly to the board, 7 total connectors I believe, plug directly in to some of those headers on the 2 boards.
Now, and if I can make a recommendation for a general strategy. hold onto the parts, but focus on getting a working complete cab. There's a discussion around here somewhere, and i put together a bullet list of things to look for when buying a dk. I meant to revisit it and make it more thorough but its already pretty exhaustive. I suggest getting a complete working cab and if anything on the cab isn't to your like as far as the parts you already own, then swap them out and sell of the spares...or hold onto them and be a hoarder like some of us. I think that cp is going to be your prized piece, and as KongTower mentioned, thats often the part that is not original or missing parts. And of course a spare dk 2 board stack is never a bad thing to have. At the same time, working 2 board sets sell for $225-$250 All day long, more to the right person. A good complete working cab can be had for $500-$600 if your patient and know what to look for. Yes, they go for more on average, but good deals also come up often enough. I mean shoot, you found those prized possessions at a garage sale...
Well, I hope I gave good info. Good luck in your search, would love to get updates.