I bet on option 1. I think Quirrell has been expecting Dumbledore to be hiding in the mirror. He said:
[...]I saw the Headmaster missing… but for all my magic can tell me… he could be in another… realm of existence..
Quirrell could be lying, but Dumbledore being hidden near the stone seems reasonable too. Also Quirrell said (in parseltongue):
I have plan to sstop even sschoolmasster, if he appearss before uss.
Appears before us. To me the verb appear evokes images, like in a mirror, more than running into people.
I bet on option 1. I think Quirrell has been expecting Dumbledore to be hiding in the mirror. He said:
Quirrell could be lying, but Dumbledore being hidden near the stone seems reasonable too. Also Quirrell said (in parseltongue):
Appears before us. To me the verb appear evokes images, like in a mirror, more than running into people.