Please don't take this the wrong way as in all seriousness, I hope you will follow solid advice.

First, don't try and re-engineer a mini apprentice. There is nothing wrong with the stock setup. The motor failing that soon sounds like a fluke to me. I have used and continue to use the e-flite brand of motors from Horizon Hobbies with no failures with hundreds if not thousands of flights. Do yourself a favor and use the replacement motor they sent you. One easy way to get more power out of the stock motor setup is to use a battery with a much higher "C" rating.Lots of beginning fliers in my local club use the Apprentice and I have witnessed a lot of punishing landings and some really ugly takeoffs that should have never left the ground but I have not seen the props breaking as you have in your experience.

Second, I agree that sometimes the stock propellers are not as sturdy as we would like them to be. You are going to experience a bunch of grief and frustration trying 3 and 4 blade props on that plane. My recommendation is to find an APC prop of the same or similar diameter and pitch as the stock prop. These are far superior props to most of the black plastic ones that come with a lot of the small arfs.

With this small a plane, you are going to have issues with it trying to fly off of grass. It really needs to have a hard surface. Surely you can find a place somewhere in your area.