This isn't at all what you're asking, but how do you think the ADU will affect those renting the main house and how much they'll be willing to pay? It's on a full acre, so I assume there's some space between them, so that's a plus. However, since the main house is 3,300 sq ft, I imagine they're paying quite a bit in rent and like their privacy, so an ADU could be a big negative. Are there trees or bushes between the two houses? Will they share the same driveway? Do you have current long term tenants or do you expect a turnover?
For what you are asking, make sure you have umbrella insurance. For deductions, is all the construction for Airbnb/rental or will you be living there at all? If it's all for business, then all your expenses are business expenses. I think you could probably furnish it for less than 7k if you buy used, and it would be silly to purchase everything new.
I don't have experience with the LLC route, but financing might be dependent on it being under your name personally. A bank won't loan money to an LLC without sufficient assets, which I think usually means a person has several rental properties lumped together into one LLC. Maybe the financing is from a HELOC or cash out refinance of your rental property, which I suppose could potentially be made into an LLC, maybe?
