I do not know every market in the US but typically speaking popular vacation areas which drive up rates also have high real estate values. 

The typical approach is to buy the houses individually then move them to an LLC.Have you talked to asset protection specialists?

Speaking in generalizations as it really depends on the deal, but SFHs won't have as much cash flow as a multi typically. 

Several methods come to mind for the ADU.   1) a regular or periodically adjusted flat fee (based on an estimated usage); 2) factored into the rent (essentially the same as a flat fee); 3) percentage proportionally based on sqft. 

Do you mean that my monthly return would not be as high since the homes are typically cheaper?

So septic costs are primarily based on how many bedrooms the house has as they typically require 100' of drain field for each one.

Even if you buy a property that was built in 2024, that roof will need replaced, typically in 25 yrs if new.  

However, an essential exception emerged in the 1990s, allowing specific rental income to be categorized as non-passive.

Sometimes HVAC contractors will jack up the prices in the winter because they are typically busier and/or they feel the homeowner might be a little more desperate to get the heat on than they would be in the spring/summer months.

Did you ask the gas company what the typical bills have been costing?