Does a request for a Pop-Up IFR clearance constitute filing a flight plan?
I think Porcupine911 nailed the first question perfectly with the reference to JO 7110.65W: A "pop-up" IFR clearance counts as filing an IFR flight plan.
ATC considers it an "airfiled" flight plan (VFR-to-IFR), and the controller talking to you will take at least the bare minimum information necessary to enter you into the ATC system and generate a flight plan / strip for your aircraft.



Your second question is a little more complicated, but the short answer is that pop-up IFR clearances aren't "tacit authorization to omit information required by 91.169", but controllers are busy people and don't have time to key all that into the computer when they're supposed to be working traffic.  

The longer version is that what you probably should be doing to get an IFR clearance in-flight is calling the local flight service station over the radio and air-filing your IFR flight plan with them, then checking in with ATC (who will have all the information Flight Service took from you and entered into the computer, without having to tie up their frequency interrogating you for it). 

The FAA (or at least the ATC organization) is not without a heart though: They understand that if you're calling them for a pop-up IFR clearance it means you're in a bit of a situation (like IMC closing in on you with no good "out" that will allow you to remain VFR) and you need to get into the system before you hit the ground, a building, or another aircraft.
Giving the controller the bare minimum skeleton of a flight plan allows them to get you into the system and under positive ATC control to ensure you're operating at a safe altitude without any traffic conflicts, which satisfies the first of the three ATC operational criteria ("Safe") -- the other two (Orderly &amp; Expedient) are a lower priority and can be dealt with later.