You do not need a Microlight Rating if you have a UK-issued JAR-FCL SEP Class Rating - it is included in the privileges of the SEP Class Rating once you have completed appropriate differences training. Check LASORS.
Except for the NPPL, you cannot run 2 UK-issued aeroplane licences concurrently. The 'senior' licence subsumes the 'junior' - all ratings are to be maintained on the senior.
I concede that many R examiners don't do much in the way of instruction - just make the simple task of talking on the wireless needlessly complicated. But many certainly do a fair bit of decent practical instruction.