Not so far as I know.
2) Can I conduct the training for such folk using my EASA PPL and then recommend them to the examiner? Or must a FI do such training?
So far as I'm aware with an EASA PPL + differences + CRI you can do this instruction.
3) For LAPL(A) and PPL(A) licence holders wanting to fly microlights, I understand that I can conduct microlight differences training and the licence holder doesn’t need anything more than my signature in his/her log book?
That is also my understanding.
4) If I want a NPPL(M) myself, is it correct that I'll need to do a test?
No idea, but why would you want one? If you want a national licence, just do the form and £42 (or whatever it currently is) payment to CAA to get a UK PPL (not the same as an NPPL) which gives you a non EASA but ICAO compliant PPL.
5) For the NPPL(M) holders, is differences training to add SSEA sufficient without test and can I do it?
No, they need an exam and a skill test. The training also needs to have been within a training organisation, under whose auspices you need to have issued a certificate of completion of training.
6) Anything else I should know before I get asked to sign a log book?
Yes - make sure you're fully familiar with the BMAA instructor and examiner guide, including the standard microlight check mnemonics; also the contents of Brian Cosgrove's "Microlight pilots handbook", as that pretty much defines the level of knowledge microlight pilots, trained only as microlight pilots should have.
Also get on the BMAA's list of instructors teaching on microlights. The emails tell you when anybody is looking for instructors, and any updates on the regulations - plus the BMAA's annual instructor seminar is about ten times as useful, for about a tenth the cost of anything I've ever attended in the SEP/"group A" world. Aaron Bliss at the BMAA HQ in Deddington is the chap to talk to about that.
G