1) I use an immersion chiller on 11-12 gallon batches. How cold your water is will play a major part in how long it will take to chill. I would think the water stays fairly cold in Michigan. If you agitate the IC or whirlpool during the chill it should be fast enough.
2) No harm in splitting up batches except that it can make yeast starters more cumbersome. Splitting a yeast starter to pitch equal amounts of yeast in multiple carboys would be a real PITA.. but it allows you to experiment to compare different yeasts on the same beer in different vessels. I really wish I could buy a Brewhemoth
right now. 22 gallon conical for about $430 shipped is a screaming deal, but SWMBO isn't working right now and things are a little tight.
3) I do 10-12 gallon batches fairly regularly. My sig is way out of date. Plan your sculpture do be able to chill your boil and drain or pump to your fermenter without moving your boil kettle. Fermenters with over 10 gallons are very heavy, if you can pump to your fermenter or drain to it without having to move fermenter afterwards you will be happier.
You will save a ton of money buying base grains and hops in bulk. See if your local brew club ever does group buys for grain. Our local club is doing on this week and I'm getting 55# of premium Marris Otter for about 80 cents per pound, and 55# of german pilsen malt for about 75 cents per pound.
My BK is a 15.5 gallon keggle, FWIW. Foam control drops are your friend when boiling 14 gallons in a 15 gallon pot.