Nice to hear the problems is solved. I have bumped into supressors like these in pedals as well as near the motor. I quickly adjusted to knee lever speed control, going from years on various Bernina machines to an old Singer 99. I also have an Elna Supermatic with a knee controller that's in regular use. I haven't owned a model 15 yet, but I know replacement bobbins can be a bit of an issue. If you suspect there might be something odd about the bobbins you received, it just might be. It happens all the time, and bobbins should behave problem free.

If the lacquer and decals are in good condition the 66 is worth fixing up, a missing plate or other loose part is usually not expensive. It's awful really if it's just tossed out because it's dirty and need of oiling and cleaning. Very rusty machines I hesitate to take on, even when thrown after me.

I don't know about the 66, since it was introduced in 1900 and are different to older model like 15, 27, 28, but some had much heavier hand wheels. For the older machines there are more than two different types, even though the later variants often had interchangeable parts.

There have been a few comparisons of the 15 and 201, and I'm sure the stitch quality on your 15 can be sorted out. The beige 201K23 will look a bit out of place in a treadle cabinet ca 1900?

The beige 201K23 is a bit different is some ways, mine takes an inbetween motor belt size to any of the black 201s (small and large hand wheel). I know very few came with the spoked hand wheel for hand crank and treadling. The castings of the aluminium body is not the same as the cast iron version, and it's possibly so with the hand wheel. By the mid 1950s most machines sold were setup with motor and lights by far (there isn't much detailed info on these), I have seen a single aluminium bodied 201 hand crank with all original parts. From the web I have seen some use a black, spoked, cast iron hand wheel from an older machine, but I don't know if it needed alterations for the aluminium body to accept it.

A model 66 is a nice machine, much the same drop in bobbin case, occilating of course. Cleaned and well oiled it's a smooth machine capable of exactly the same as a 201. The 201 runs a bit smoother, three sets of meshing gears makes a difference, but in strenght and capability they are equal. The 201 has a full rotary hook and race, and a revers lever. If it was me, I would keep the 66 in the treadle, and have an additonal 201K23 with motor and lights. The hand wheel on the 66 needs oil, when the stop motion screw is off, it's easier to get oil all the way along the axle there; it's very hard to reach this part from the oil points and it's sometimes stuck. Oil and wiggeling should loosen it; and I imagine other parts of the machine might need some extra attention too. It can take several days of cleaning, repeated application of oil, running the  machine and more oil to get it right again.

...I think some of the stitches are a bit dated but have a play and get them used.

They aren't really dated, but more the result of a cam stack with single layer cams, which limits the stitch pattern compared to models with double layer cams. Elna and a few German brands (Adler, Gritzner, Phoenix) had this feature in the 1950s, and with it came stretchy seams, over lock type stitches (single thread). Japanese brands caught up on this in the 60s (copied a Gritzner machine and gradually developed their own take on it). Bernina didn't have this feature until the 1980s, but they had the smoothest running, most well behaved machines compared to any, but the stitch selection is narrowed down to mostly utility stitches and a few decorative ones.

I haven't used any of the more decorative stitches on my 730, mostly the decorative stitches and the buttonholer. It has the best satin stitch.

Spend a bit of time cleaning and oiling it; the top of the machine is hinged in the back and lifts up; the top of the freearm comes comes off too.

I have polished my Berninas with Auto Glym Super resin polish to protect it a bit (the liquid type that dries to a white film before its' buffed off). The finish on these is rather delicate, but staining from oil and grime usually cleans up perfectly.