I used to fly a flexwing microlight and these days instruct in mainly PA28s.
In my opinion there's no reason why the two shouldn't co-exist quite happily and safely.
The speed difference can be taken care of quite easily by flying different size circuits so that the timing around the circuit remains similar - just like you might have to do for a fast conventional aeroplane compared to, say, a C150.
There's a bit of argument over how steep a PA28 approach should be on this newsgroup and others from time to time, but the typical teaching in the UK is to fly the approach with power a bit shallower than you suggest at about 3 or 4 degrees. Microlight approaches are typically flown without power and so are rather steeper - but probably not as steep as your post suggests unless the microlight has the glide qualities similar to a brick.
Anyway..., the point I'm trying to make (badly) is that all these differences are not really so different and one can experience similar sorts of differences between types and piloting styles of conventional aeroplanes in the same circuit anyway.
In this case, if the facts are as reported, then the PA28 pilot made a mistake and excercised bad judgement. It's hard to believe that a good forward thinking pilot should make this mistake but people do make mistakes and there's no point pretending otherwise.
Given that you both survived his mistake, the remaining unresolved problem as reported is that the pilot is sufficiently arrogant as not to acknowledge his mistake and so not to learn from it - I consider that to be unforgiveable and you shouldn't feel any guilt at all in pushing the matter. It's just possible that you'll save someone elses life by doing so.
I would suggest that you file the airprox.
Also, you infered that this wasn't an isolated incident at the airfield - is it a general problem or is it the same offender each time. If it's the same offender then I would suggest that you make sure that the airfield operator is aware of the incident so that they can, if they choose, either get the pilot concerned to think a bit more or to fly from somewhere else. A fatal accident could be really bad for the airfield operator's business.