I would not rent a house with 4 beds and 1 bathroom on another floor unless I had no other option, or was just getting started in life.I’d comp this below other similarly sized assets, but it’s all about the numbers.

A home equity loan gives you an option of having fixed debt which is safer if you plan on using it to buy a long term rental buy and hold scenario.

In fact, even no one else and lose another month of rental income would be a safer option than to allow this person.

It's becoming more and more common for people to find longer stay options on Airbnb, especially since Airbnb started promoting longer stays last year.

Do manufactured/mobile home dealers provide seller financing option to buyers? 

First option is to sell the house immediately and utilize those funds (~100k) to supplement the cost. 

Hmm... seems like framing a floor vs cement could lead to moisture issues (or maybe not otherwise why would they allow that option)...

I've thought of a HELOC but looking for a longer term solution.Are there any other options?

In the middle of those two extremes might be a small business with 3 years left on their lease, but they've made improvements to the property that aren't cheap to remove, such as perhaps a dental office (dentists might be pissed that I put them in the mid-risk category, but all the dentists going out of business in 2020-2021 taught us that American consumers apparently regard dental care as an optional and "luxury" spending item). 

Same for backyard - even if it's small, cutting it in half with a fence makes it easier for both to enjoy a holiday barbecue in their own space.These are low-cost options that will help create a better relationship with tenants.