Disclaimer: I am not a CPA, nor a Lawyer.
However, I am an owner/PM of a Vacation Rental in a Colorado mountain town.
I started out with a dream of a second home in the mountains too. Then toyed with renting it out to some family friends after I remodeled it. This quickly turned into a full time business for me. And by full time, I spend about 4 hours a month on it and it's much more lucrative than my long term rental in the metro area.
I always assume that the IRS knows everything. It's just the safest route, in my opinion. However, you bring up a good point about those 14 days not including maintenance. I find that there is always some level of maintenance to be done, since a vacation rental has a lot of traffic. It's never anything major, but the little things add up. If I get up there once a quarter, there are usually at least 3 or 4 small things that need fixing. So, if you wanted to be there more often than that, you could certainly find valid maintenance reasons to visit.
As for how many days it was rented... this should be very easy for anyone to find out. I use VRBO and AirBNB to book my rental. I also have my own web site with calendar that reflects bookings. Those calendars are all tied together so I don't double book. Therefore, it's easy to see online just how many days I'm rented.
And signatures/documentation when you rent it? Yes, this is mandatory. You don't want to allow ANYONE to stay in your property without a lease in place to protect you and your property. Even if it's just the short term rental agreement on file with the vacation rental sites, which people must check a box to prove they've read and agree to before renting.
Again, my recommendation is to assume the IRS knows all. There are plenty of real benefits that the government gives to RE investors, so no need to hide anything. Consult a CPA for the full run-down of good reasons to rent this out and take full advantage of the associated tax breaks.
Ah, the old LLC question... it's more than just an LLC question though, it's also about how you would run this business, as a sole proprietorship, partnership, corporation? S-Corp or C-Corp is essentially what it boils down to most often.
You'll want to consult with a lawyer to help you make the best decision for you on this. But here is what works for me - An S-Corp LLC, with the property deed in the name of the LLC.
Why? Because having the business/property owned by an LLC gives me a level of protection from lawsuits.
Is it unassailable? Can they pierce the corporate veil?
Well, that depends on how much work they want to do. I'm also looking at setting up a land trust for the deed as an extra level of anonymity.
As a landlord, it's not if you'll be sued, but when. I feel it's best to do as much as possible to protect myself in advance. Including leveraging the property so there isn't a lot of equity in it to tempt people to try and take it away, as well as making it difficult for them to come after my personal assets.