Hi Blackcat
As other will attest I am a great fan of dobs. I did a lot of research before buying my dob and remain convinced they give great value for money.
You have asked some really good questions that indicate that you have done some research so I will try to answer as best I can
As far as taking photos is concerned you can take photos of some brighter objects EG moon and planets with the SE, and for that matter with the dob, but don't expect startling results. if using a DSLR, the afocal method is probably the way to go. If you want to attach the camera you would need at planetary camera or webcam style. For DSO imaging neither scope is really suitable.
Not sure about the operating the SE without power. Bintel do a power solution ( see
http://www.bintel.com.au/Accessories...oductview.aspx ) which I use for the dew protection on my scope and it is simple and reliable.
In light polluted areas, aperture is usually a good solution, which leans towards the dob. Contrast is mainly affected by the centre obstruction which tends to be larger in the Cassegrain scopes. Sharpness is more to do with the eyepieces. Both scopes even with cheap EPs will be fairly sharp in centre. But you will notice that they get worse towards the edge of the field. Good quality EPs will give wider sharper fields.
So again a dob will leave you with $$$ to spare on those EPs.
In the future as you said the dob can be adopted as an imaging scope, so that is a plan.
Good luck with your decision
Malcolm