Filter Sets

Yes - even if you might be able to reach focus without the 5mm extension, the sensor would be so far away from the distance for which the lens is designed, that the quality would suffer.
So, we highly recommend the extension.

This depends on the connection to the pillar below. The short pillar itself is made of massive steel, this part itself can easily handle ~100kg of weight, the question is how solidly you are mounting it onto your pillar.

We do have four different Nikon T-Rings for different camera models:
https://www.baader-planetarium.com/en/catalogsearch/result/?q=nikon

Regarding required extensions, we highly recommend to look at our Digiscoping Instruction Manual:
https://www.baader-planetarium.com/en/downloads/dl/file/id/444/product/0/digiscoping_camera_adaptations_for_afocal_photography_and_eyepiece_projection.pdf

Unfortunately a 50 mm filter will not fit into our cells because the connecting male/female (standard) threads measure 48mm as outer diameter - and consequently the female side must be smaller then that in clear diameter, otherwise filter cells could not be combined. Accordingly the largest glass diameter could be 47.5 mm and the smallest possible size would be 46.5 mm, whereas this already will be problematic because the filter substrate will need some camfer (ground bevel) in order to protect the glass from chipping. So in reality the filter should not be smaller than 47 mm.

Unfortunately the M60 ClickLock is one of the few where this is not possible