3 bunnyhops, waggle your ears and a quick grope of Fern for you, Gus!
No, the NST is taken before the NPPL Q X-C, not before 'any' NPPL solo navigation.
The medical declaration for a NPPL has nothing to do with HGVs - it is merely conducted in a similar manner, i.e. with your GP rather than with an AME.
The NPPL NST is regarded as a Progress Test towards a JAR-FCL PPL; it does not prevent you from completing your training under the JAR-FCL system if you wish.