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OP, I split your post to form a new one here. I had to edit the title, but changed nothing else. If you'd like to change the title, click "Options" and then "Edit Message".
There could be several causes for the drop. First off, did anything else fall off between then and now? Did you pay off any closed credit cards or close any since applying for the mortgage? Did anything fall off your credit report since applying for the mortgage?
Throwing out ideas, but if you closed and paid off any CCs there could be a drop due to that. If any old accounts fell off, then you could also see a drop due to the loss of history and changes to your average age of accounts. Added collections can certainly hurt. IME, I've lost anywhere from 60-100+ points when a new collection gets added to an otherwise clean report. That could certainly be the cause. Adding the mortgage could ding you due to the new credit but the impact of that would likely have gone away by this point.
Aside from the collection, you could benefit by adding a CC. The limit won't matter at all, but it'll help your mix of credit and I bet you'd see a nice gain. I personally would aim for a secured card to start. In fact, you really wouldn't have to use it. Just buy a pack of gum or cup of coffee and pay it in full to keep out of debt and shelve it for the following 2-3 months. Lather, rinse, repeat. I bet if you added a CC, you'd gain quite a few. I'd aim to get rid of that collection first, though.
