Since many an extract brewer has waited overnight to let the wort get to optimal temperatures, I think you might be being over-reactionary
Really, risk of contamination is pretty minimal IF you had properly sanitized your fermentor and you have it properly sealed. My prediction is that your batch will turn out excellent!
As for pitching temps....your yeast manufacturer should have details about what ranges are optimal. Ale yeasts are going to not like 50 degrees, obviously, but lager yeasts will do fine. Though I imagine most yeast strains might just go dormant at 50, but won't die (IE no longer be viable).
One suggestion: if you don't have a floating thermometer, get one! I've always used one, and it's great for keeping abreast of your wort temp. You can either sanitize it while you're doing your boil, or with whatever sanitizer you use for your fermentor.