Okay first one is easy enough, you need the mixing valve or anti scald valve as with a sidearm the water temp in your water heater can actually get much hotter than it could if your using a flat plate heat exchanger, say 170+ degrees. The anti salad or mixing valve helps to ensure you never get hot water out of a tap any hotter than what you set the valve for regardless of how hot the water is in your water heater. I’ve never seen it suggested to cut the tube off in the water heater before though. I’ve also not seen it suggested to T your cold supply into basically the drain of the water heater.
Central Boiler really does have quite detailed schematics on their website.
You’ll only need an expansion tank if its a closed boiler, since I’m assuming like 90%+ of the outdoor boilers on the market Polar’s are an open system, you shouldn’t have any need for an expansion tank.
Lastly, if you go the route of using the valve at the furnace heat exchanger, then yes it will need 24 volt from some source to open it to direct water thru the HX (heat exchanger).
IF:
A: you don’t plan on running your fan on the furnace 24/7 or
B: you don’t plan on running year round for heating your household hot water
then you really have no need for a diverter valve at the furnace heat exchanger.
A more reliable way to do it is shown here using a secondary loop, look at the second picture to get a little better ideal about installing the t’s for a secondary loop:
http://outdoorwoodfurnaceinfo.com/forum/index.php?topic=8522.0