I was attempting to roast a head of cauliflower with olive oil and seasonings so it comes out crispy. Following the instructions, I roasted it for 35 minutes at 450°F (the recipe said 30 minutes, but it didn't seem done), stirring once halfway through. However, after they cooled, it appears that they've steamed instead of roasted. My guess would be that I put too many on the sheet. Can I stick them back in the oven for another try? If so, for how long, and should I use more oil? 
No, cooking them for too long will make them mushy, whether they’re roasty or not. You could try oiling then a bit and sticking them under the broiler for a couple of minutes (watch them closely!), but i wouldn’t cook them longer than that. 