I'd love to read an essay about the JLR Pathfinder DoIP service system.
Do you HAVE to use a Panasonic Toughbook?
Which DoIP devices are suggested?
Which subscriptions do what?
What are the datalogging or PID display abilities?
Where to find a list of PIDs?
Unfortunately I was mainly the mechanical aspect of servicing as opposed to software/fault diagnosis... however I do believe the Panasonic Toughbook was standard issue to JLR techs for fault-finding.
Most of the answers to your other questions will be available through the normal channels - ie if you are a registered dealer then you'll be able to obtain all of the diagnostic software required for i-PACE. (As a home mechanic myself I appreciate this is probably not what you want to hear!)