Can you boil milk and let it fully cool then bring it up to 100 degrees F for yogurt?
Yes, you will be fine.  As stated in this question, the main reason to heat milk for yogurt making is to improve texture.  Heating it twice should not be a problem.  It would be interesting to know if twice heated milk (heated, cooled, and reheated) has an impact vs. the traditional heating, then adding the culture at the correct temperature.  There is also useful information in this question and answer, as I was trying to understand the variables in yogurt making.  If you are interested in playing with the texture of your yogurt, you will have more information here than you need!  If you don't care so much about dialing that texture in to your liking, and just want any yogurt, your process will certainly work.