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Odd. I guess "yes" to all your questions. I haven't done it before but I guess you can roughly measure the focal length of the primary by setting up a torch shining on it a bit of axis, then see how far from the surface the torch light comes to a rough point. Might be worth doing as a check, then compare with measurements on the tube unit.
There is absolutely nothing else in the light path - no lens or similar at the bottom of the focusser draw tube?
Mounting the mirror several centimetres up the tube shouldn't be a problem, as long as you can get your fingers onto those three wingnuts which will be used for collimation.
I guess a question arises as to whether the secondary is now doing a good job of capturing all the reflected light - ie. is the outside of the light cone from the primary missing the secondary all together? OK, we need the experts' views now.
However, before driling holes and cutting tube to length, best to stop and think and get some other eyes/brains on the job in case something is happening to cause this.
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