rjm1982,
Ok, you're more than likely not going to hurt the motor, so relax. Backfiring a 4-stroke will definitely throw a prop, but you know that by now, just feel lucky that you didn't do it in the air and loose a prop and spinner. I'll try to answer all your questions, but if I miss one please let me know.

For break-in you can take the spinner and backplate off. Tighten down the prop nut as tight as you can get it. Throw away your little 4-way wrench and get a box end wrench and a glove. With the 4-way you'll never get it tight enough. Use a box end wrench to tighten it, oh yeah, the glove is for the hand holding the prop so it doesn't cut your fingers. Don't worry, you're not going to be able to tighten it enough to damage and threads, but that's why you should use a box end so you don't slip and round off you nut.

After you get the prop tight, prime the motor but don't get too much fuel in it. Too much fuel (or being too rich) is the main reason for a backfire. To prime the motor, open the throttle to full (make sure you DO NOT HAVE YOUR GLOW STARTER ON), cover the end of the muffler, rotate the prop until you see fuel pulling into the carb, and then turn the prop through another compression cycle. Then take you finger off the muffler and rotate the prop through a few cycles to pull the fuel into the motor. Then connect your glow igniter and try again.

A trick you can use for starting a motor without a spinner is to pull the rubber insert out of your electric starter and turn it around. It will have a small hole in it that you can fit over the prop nut and rest the silicon up against the prop. Before you do that turn your motor over in a clockwise direction to just before it hits compression, then put the starter on it and try to start the motor.

Hope this helps.