I've been leaning towards theory #2, actually. I'd theorized previous to this episode that Donna would part ways with the Doctor either A)by slipping through a dimensional rift, or B) because the Doctor goes missing, and his Companions must band together to find him.
After watching "Silence in the Library", both situations are plausible, and could happen concurrently. River Song Theory #2 is more in tune with how the classic series went. On rare occasion, the current Doctor would meet his past selves (for example, a villain took each incarnation out of the time stream, and it caused all sorts of chaos when they were brought together). The universe nearly imploded and reality had begun to break down. Much like the rift in Cardiff, these tears in reality allowed things to slip through. It's quite possible that in order to seal this potential leak, River is the price.
no subject
After watching "Silence in the Library", both situations are plausible, and could happen concurrently. River Song Theory #2 is more in tune with how the classic series went. On rare occasion, the current Doctor would meet his past selves (for example, a villain took each incarnation out of the time stream, and it caused all sorts of chaos when they were brought together). The universe nearly imploded and reality had begun to break down. Much like the rift in Cardiff, these tears in reality allowed things to slip through. It's quite possible that in order to seal this potential leak, River is the price.
*is too tired to explain self fully*