Quick answers to your questions.
The regulations do not (appear to) specifically state the following, but they seem reasonable extrapolations
* A CRI can instruct for transfer between NPPL(SSEA) and NPPL(M), in either direction, but a separate examiner will be needed for the NPPL(SSEA) skill test, and the CRI could not authorise any solo flight before issue of their NPPL(SSEA)/
See
http://www.nationalprivatepilotslice...20REV%2008.pdf
This specifically mentions CRIs and what they are entitled to do towards NPPLs.
* A CRI with appropriate experience and differences training could train for the 3-axis/flexwing differences training on microlights in either direction.
Correct. This is in LASORS Section H0:
"Holders of JAR-FCL licences which contain appropriate Instructor Ratings (and Examiner Authorisations) may exercise the privileges of the ratings/authorisations included in their licences on Microlight aeroplanes
and SLMG’s but shall first undertake any necessary differences training."
* A CRI(SPA/SEP) could provide a ME-only pilot with the necessary instruction to obtain/return SEP currency.
Correct. You will be training a licence holder towards the SEP Class Rating.
* Does a CRI need a CPL to get paid? Presumably they do, but the regulations seem to imply the opposite.
Yes, since it is commercial activity.
* Can a CRI teach aerobatics?
Yes.
HTH.
ifitaint...