Thank you, everybody!
As to the crook on the top of the Kubota hydraulic space... so THAT'S what that thing was! (but that was my old tractor not the current one).
Yes, I did get the new filter to seat. I don't know what kept going wrong the first several times. I was getting a pretty good stream draining out, like the gap must have been at least 1/16" and maybe 1/8". It would rotate several turns and then stiffen considerably, which felt like it was seating, but visually it obviously wasn't. The old filter gasket kept sticking to the tractor rather than coming off with the filter, but I knew that and was dealing with that. I felt for some other obstruction but never found it. Of course I was frantically trying to stem the tide, so it wasn't a great time for thoughtful examination and testing ideas. I'm stuck clueless as to what the problem was. However, eventually it worked, and worked perfectly well as I'd expect with no question about it, and I wasn't about to experiment any further.
About the vacuum cleaner idea, I wonder if the vapor over the oil could be combustible, ignited by the brushes in the vacuum cleaner motor. But I think if it was, that's what most of the stories would be about.
So now I have a NEW PROBLEM. The loader hydraulics are very spongy. I often park with the loader close to the barn door, and just walk across the blade edge if I want to take the shortcut in my fairly small storage area. And I generally get the bucket pretty level when I park and shut down. But yesterday I noticed the bucket was curled a bit and still resting on the ground, and when I walked across it, the loader arms and the bucket were both flexing significantly under my weight. Tractor's new, with nice tight joints, but I could feel with my finger that my body weight was moving the cylinder rods in and out a good quarter inch. Now, I could stand to lose a few, and keep some of my ballast right behind the steering wheel, but this is definitely off!
What I had done after my refill, without having let anything hydraulic move, was to run for a while until the engine was warm. I wanted the air in the filter and lines to get pumped into the tank, not into any cylinders. Only then did I cycle the loader arms, the bucket, and the 3 pt. But I think I must have air in there all the same. I did also notice an unfamiliar, faint, high pitched whine when I started, before I used any hydraulic motions. So I've been reading up on getting rid of air. Any advice would be VERY APPRECIATED!