There seem to be a couple of threads mixed in here.
Should you be able to change instructors? Of course in the end it's your money bit of choker if you've pre-paid and the school you are with has few instructors.
Should instructors be paid more? I'd subscribe to that, however flying is already very expensive. I'd go another £5 or £10 per hour (ouch) - I think prehaps hours builders should be given a different oppertunity rather than FI to gain hours. Make instructing a profession or as had been said FI rating only arrives at 500 hours. Prehaps the NPPL will polarise this further.
On my landings (forgive me I'm only at 7 hours or so and with todays WX unlikley to get more) they are getting better. I've been advised that I should be able to complete the course in 45 hours - but I want to graduate and fly safe (my life and my familiy) so if it takes 70 or 80 I shan't bleat.