There will be hundreds of viewers familiar with the 125!
I've based my answers on the 125-700 although with the Viper you may be discussing earlier models, Series1 to 600?
Yes it does have to be disconnected for towing.The Torque link itself is not disconnected there is a vertical steering disconnect pin that connects the torque link sleeve and steering sleeve (a picture explains all). It's very easy. Once disconnected I seem to remember you could go to at least 90 degrees.
Pass on the Viper question! - I think I only changed 2 in 30 years for turbine overheat. I had to persaude the CAA to let me take the type rating oral without doing an engine change.
Hydraulics - Very easy with no sophisticated tooling required. Both Main and Auxilliary have a screw filler cap that when removed you can just pour the fluid in (or unofficially from something like a pressurised bottle). Level indicaters are visual but the pipes often get discoloured.
Hydraulics - Nitrogen quite easy to access and top up but obviously need some tools and connectors
Cabin Door - Not clear what you are talking about. Internal doors or main entrance door? Internal doors are just a simple catch (usually open on a heavy landing). Main entrance door is a plug type and depends on the aircraft model as some early models had the "up and over door" later ones had the combined door and stairs. The door pivots on two hinges. Closing the door lifts the door and drops it into position using spigots and guides 5 altogether if I remenber. Works well but can get out of sequence occasionally.
Flying controls are cables and pulleys
Deicing is by TKS fluid through porous panels on the leading edges - OK but messy.
Even for an overview you might need to be more specific as to which series. Anything more detailed and I'd have to dig out the old books (I pre-date CD's!!)
Hope this helps
Mike Echo