The large radius bends are for water flow. Many of the coil instalations use gravity or thermosiphon flow to circulate the water. This requires as little resistance to flow as possible.
I don't think a tempering tank would work with a boiler, only with a domestic hot water tank. Use the boiler itself as the 'tempering tank'. Any captured heat will be in the boiler to use the next time the circualtor pump calls for heat.
You will be able to recover some heat from adding loops into your stove, but I don't believe you will be able to recover enough to heat the basement. I don't know how big the rooms are or how well insulated they are in the basement. You don't want to try to heat rooms with concrete floors and walls. The concrete absorbs huge quatities of heat.
If you hook one coil to the boiler you will get some residual heat in the boiler, it would only be a fraction of what is needed to heat a basement though.
If you hook one coil to a tempering tank in the cold water supply side to the domestic hot water, this will provide quite a bit of heat overnight for hot showers in the morning.
There just isn't a lot of heated surface on the coils so the amount of heat captured by the water is not great. But if it keeps recirculating all night or all day long, then you can bring a 40 gallon tempering tank up to 180-200*, it depends on so many variables. In the winter, with the stove really cranking, you will get a lot more heat from the coils. In the fall and spring, not so much 'cause the stove will be idleing a lot of the time, if you turn up the stove to get more hot water, you will get chased out of the house by the heat.
So if you get creative, you will gain some hot water from your coils, but I just don't believe you will see a huge reduction in oil use. Depending on the cost of the coils and instalation costs it may not pay. There was a thread about a year ? ago titled 'was it worth it' author wenchris I think try a search for it.
I would keep the domestic hot water coil separate and dedicated to domestic water. Keep the second coil dedicated to the boiler. Boilers should not have fresh water with fresh oxygen added to the system this promotes corrosion in the boiler. Domestic hot water heaters are designed to have fresh oxygenated water circulating through them.
Lots of variables and options, Experimenting will tell what works