Can I service an Avid Single Digit brake?

Is there anything I can do - besides reset the spring to a different frame boss notch to create stronger cable/lever return?

Is it worth while to disassemble the parts of the brake on the frame/fork boss, or just sort of hold it together and reset spring?

So what's wrong with it? There's not much to go wrong with v-type brakes, and the spring should be strong enough that under normal use it will not fail / loose strength unless it's been bent.

For servicing, there is not much in there apart from the spring, has the brake lost it's adjustment screws?

Perhaps the post mount on the frame/fork is dirty or damaged? Usually removing the arms from the post, cleaning both and applying some lube, and putting them back together will solve issues with the arms not retracting evenly. Sometimes sanding/filing the post lightly is needed if there is corrosion or a burr there. To test, if you remove the wheel and release the spring tension on the arms, do they pivot on the posts smoothly? They won't work properly unless they do.

Avid SDs don't pivot on the frame/fork post. They pivot on their own internal post. You can pry them open for cleaning and relubrication, and if you do, also consider drilling your own grease-injection port through the arm and its internal brass bushing. I did this on my foul-weather commuting/shopping bike.