A microlight is an artificial definition, two seats, lightweight - light regulation.
You can build bigger heavier aeroplanes, but they're not microlights, simple as that.
Re: night and IMC. There are legal mechanisms that would allow you to negotiate this now through "special conditions"; the reality is that it's a lot cheaper and easier just to buy an aeroplane with a CofA.
There's never been a "no abroad" ban, it's just that the aircraft aren't ICAO compliant, therefore you need approvals from the countries you're flying to. Again, you could circumvent this by following ICAO procedures - but why on earth would you bother when most countries will let you in on the basis of a few faxed documents.
G