There are three big things that stop many people from running their coal boiler through the summer.
-Their chimney draft is poor and they have problems with outfires.
-The radiant heat from the boiler makes their house uncomfortably warm.
-They don't used enough hot water and/or their cost of coal makes running the boiler for DHW uneconomical.
Then there is that other thing...some people simply don't want to mess with coal in the nice weather.
Ok, with that said...if your chimney performs as it should, you have insulated the boiler and piping, and running the boiler helps manage the dampness in your basement...I think it will pretty much be a wash cost wise compared to the electric tank, super dehumidifier, etc. Your boiler will enjoy working a lot more than sitting, and I think you will like the basement "weather" better with the boiler running.
15 lbs per day is probably the best you will be able to do...I burn 20 lbs per day to make DHW for a family of three, but I also have some standby losses from the oil boiler.
A tankless coil used to preheat the water going into the electric tank is probably the simplest solution, but if the tank isn't well insulated and/or you don't used much water, you will need to leave the electric turned on to keep the tank at a minimum temperature. I hooked up my brother's system this way with a foam insulated 50 gal. electric tank, and it works excellent.