1) I am very interested in DPL II. It seems to be regarded as a great enhancement. Does it work when only 5.1 speakers are connected (no rear center channel)?
We do some heavy satellite viewing on a 5.1 setup. I’ve had 2 speakers in boxes not hung for months (don’t ask) until last night, when a speaker change across my mains allowed me to get the 6th speaker on line for the first time. In 5.1 DPL II is a very very nice upgrade when feed is (very frequently on my Direct TV) PCM/stereo. Occasionally it does such a good job, I start paying attention to the sound rather than the movie. When I was viewing ‘Frequency” the other day, it was at its best. I have read that certain soundtracks do not really give DPL II anything to work with. It was I believe a Dolby Labs generated white paper, I cannot remember the particulars behind the ‘why’ this occurs just know that I have noticed it myself in my real time viewing. I source another surround mode when this situation arises.
To be honest last nights short listen to my new 6.1 configuration up from 5.1 was not a vast step forward for me in HT effects. But then I’m in a situation of working within very small room dimensions so the 360 degree increase is not as noticeable here. If anything I have to deal with over-lap rather than trying to fill in holes for the 360 effect.

2) Can you set the 950 up to only use a 5.1 configuration? If so, does it convert the 6.1/7.1 formats reasonably well to the 5.1 format?
Yes, you can configure the 950 to 5.1 6.1 or 7.1. You do this in setup, which will limit the surround mode choices available to you. Ex: You can only play a DTS-ES Discreet DVD when you system is configured to 6.1 or 7.1, playing the same DVD will default to DTS when played on a 5.1 system.

3) Can the 950 auto-detect the incoming format and set itself appropriately? If so, can you default it to use another format (like DPL II or Cirrus Extra) instead when it auto-detects?
Yes you have auto-detect or default choices.

4) Are there any important features missing from the 950 that I will regret later?
For myself, it has the most features/SPM’s at it price, and more importanly great sound. There really is nothing I’ve found in its league. Some audiophiles have bemoaned certain user interfaces. Global calibration is for all sources, Distance can be set for FL/FR Center Surrounds separately (some units offer a distance on FL/FR separately etc) I personally would not buy a unit solely on its interface options myself but the Outlaw has most bases covered.

Others can perhaps answer with more precise language than I am capable of. I'm very happy with my decision to try the 950 for myself.