From the owners manual:

The CSP2 uses a single 6N1P/6DJ8, or 6922 (which is a dual
triode) to drive both the left and right channels.
This tube is located at the front just behind the
volume control. Changing this tube will change
the sound of both channels. Because of this, it
is arguably the most popular tube to roll if you
enjoy hearing how one brand of tube sounds
different from the next.
The two other 6N1P/6DJ8, or 6922’s which are being driven by
the front input tube, are wired one tube for
each channel. The left tube for the left channel,
right tube for the right channel. These can also
be rolled with different brands and have an
equal effect on how your CSP2 sounds. Just remember
that whatever you do on the left side,
you should also do on the right - meaning the
tubes should be of the same type and brand.

To answer your 3rd question, the answer is not necessarily especially in a pre-amp where the voltages are lower, but most power amplifiers will have different type tubes for each function. I own a different pre-amp that costs more than the CSP 2 where there are two input tubes (one per channel) two output tubes that are different than the input tubes (one per channel) and two rectifier tubes (one per channel) and CSP 2 blows it away period!

To answer your 4th question I’m sure the answer is that those are the tube type that works best within the electrical parameters needed to build the unit. If I’m wrong I’m sure someone more knowledgeable will correct me.
Congrats on the purchase of a very fine product indeed.8-)

I haven't bought it yet.  I am trying to understand it.  I own a Singelpower MPX3 Slam SE (OTL/SE) and a MAD Ear+ (Transformer coupled ouput stage).   I am looking at this amp because it can also be used as a preamp.  So, I wonder how good it is at preamping?

Quiet.  Low distortion (for tubes)?

The question I am getting to is: does anyone roll other tube types (not just brands) into these positions?  For example, could one use 6CG7s as output tubes?  The Singlepower I have takes a multitude of tube types and there are 3-4 socket adapters that expand the number of combinations a lot more.

Frankly i am not familiar with tubes that use this pin set, although they can be used in the SP.

I'm just wondering how adventurous people can get with this amp in tube rolling.

Actually since I have small fortune already invested in tubes for other preamps, headamps, and power amps, I am not sure how adventurous I want to be.

I am located near Copenhagen.