Aware, the main reason Microlight instructors get paid more is that the competition isn't as fierce and the cost of running a Microlight and the associated school is very much smaller allowing for reasonable margin to run the business and yes the people who are doing it tend to be the owner of the business so the staff overheads are lower. I'm not saying Microlights aren't brilliant - they are but it's not as one dimensional as that.
The commercial pressure on a flying school to be "as cheap as possible" is very strong and the only place you can reduce cost to maintain competitiveness is to pressure the FIs. It's a vicious circle. That said at my old school the concept was that we would have as few FIs as we could get away with so that they could have as many hours as possible. They fly now between 900 and 1200 hours a year and get paid a reasonable wage.
I-M I'm not sure I think an instructor needs to have flown commercially but a wide and varied experience beyond tommies, C150/2 etc etc and plenty of cross country flying and ****t-hot handling skills makes for the best guys. For me the best instructors I ever had were former fast jet military instructors who had seen just about everything.