There is some symmetry in this situation: I don't know what you mean by "anything beyond f/16 will appear equally soft". Communication is tough!
But the answer to your question is surely "no, that's not what I mean".
Let me try again in more detail.
When speaking of a lens, "diffraction cutoff" refers to the finest level of detail that can be resolved at all. On an MTF chart, it is the level where contrast drops to zero. For an f/16 lens, that number computes to be about 114 line pairs per mm on sensor.
When speaking of a sensor, the finest level of detail that the sensor can possibly be trusted to capture is one line pair for every two pixels. This is the "Nyquist limit", and for your sensor it turns out to be 119 line pairs per mm.
119 is about equal to 114, hence my comment that your sensor has resolution corresponding to diffraction cutoff at f/16.
Lenses that are significantly wider than effective f/16 will produce images with higher contrast for fine detail, but the sensor will not capture more than 119 line pairs per mm regardless of how sharp the lens is.
Lenses that are significantly narrower than effective f/16 will produce images that are more blurred in proportion to the f-number. Your 50x/0.42, running at effective f/60, will produce an image that is very blurred on sensor, and your sensor will capture all the detail that is present in it. Your 20x/.40, running at effective f/25, will produce an image that is much less blurred, but is still blurred enough that your sensor will capture all of that image detail also.
No. The sensor that you have is fine.Is the solution to get a camera with a larger sensor or more MP?
More MP on the same size sensor would simply prepare the sensor to capture finer detail that is not in the optical image in the first place, because of diffraction.
More MP on a larger sensor might help, IF the objective could cover the larger sensor, which it probably can't. That limitation is because microscope objectives are designed to cover a sensor no larger than APS-C at rated magnification. Some of them do actually cover more, but that's an area I don't want to get into right now. The simple and reliable approach is to just pair a microscope objective with an APS-C sensor, like you're doing.
No, it asks for a 210mm tube, no lens. Finite objectives are designed to be used on empty tubes, no tube lens.You also mention that you use a finite lens and I noticed that it asks for a 210mm tube lens.
And large-NA objectives are very picky about the length of that empty tube. The standard graph of tolerance for tube length shows that an NA 0.80 dry objective can only tolerate about 2 mm deviation from nominal. Sticking this objective on your Tamron would give an atrociously blurred image, if you could get it to focus at all.