This seems the simplest, but will it cause issues down the road when I goto cash out refinance on the same property once casita is built with heloc money? 

It's been one of the simplest and most profitable investments in my RE portfolio.

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.

More often than not, I see out of state firms make errors on anything but the very simplest NH situation (heck, a lot of NH tax preparers also get it wrong). 

When these payees are shared across all projects, it's a mess tracking what expense goes to which property.What's the simplest/easiest way to do this?

Glass block and tile are probably the simplest solution.

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.

The simplest way in my mind would be to purchase the new place as a primary residence, get a low downpayment or rehab loan, and then rent it. 

The simplest way to find one is to just start calling every listing that has a purchase price in the last 2 years in your area that was on an FHA or VA loan AND has been on the market for 60+ days.

Quote from @Tom Sharon: There are many ways to address this, and I'm not familiar with your state laws.The simplest method: post a written notice on the door.