From my limited knowledge of these tools I don't think I have ever seen one that meets all of your requirements, but I stand to be corrected.
Mouse sanders are small, and as soon as you stick a battery on the back they have increased in weight and size. Corded may be easier to handle.
Mouse sanders have identical heads (two types, one is longer but the pointy end is the same) and so access into corners will be the same for all models. I would suggest a drum sander attachment (Dremel maybe) for tighter concave curves. The problem with the mouse type if that they have an edge, and can cut grooves into tighter curves.
I think I have seen a dust collection bag on only the Black & Decker cordless mouse.
Easily obtainable spare batteries for Ryobi, Bosch, and Black and Decker.
Easily obtainable sanding sheets, there are only two shapes of head, there are lots of generic sanding sheets out there.
Charger and battery. On the Ryobi, Bosch, and (I think) B&D, they come without battery, this suggests that there could be more than one battery and charger option. My tools only get hobby use, and so I have found that with a spare battery, I never really get the use of the fast charge.
Price.
The cordless are about 40-50 quid, without battery and charger; those are extra. This if for the bigger brands and will give you more chance of getting spares.
The corded are about 20 quid upwards, and you can get dust bags on several of these.