C42 has a 172kg total cockpit occupancy limit, so you need to find a 72kg instructor - I'm sure there must be one somewhere, but I'm not sure whom!
An MTOW limit is based upon a combination of performance and structural limitations and is there on all aeroplanes, and there are other limits that restrict other aircraft classes (VLAs can't go above 750kg, SLMGs 850kg...)
The 472.5kg limit is a concession for fitment of a BRS but the aeroplane has to take all of the regulatory flight loads at that higher weight. Looking at the TADS, it looks like the C42 was probably just over-engineered enough that it could be done as a paper exercise - but I'd not rely upon that being true for all 450kg microlights.
G