I've often found that just going right to the expert in the space is going to be the most direct source of info.

How big is the space they are renting? 

Tools like this helped us determine that retiring in the Memphis Metro area was good for us and has helped form our investing strategy, mainly SFR and some commercial spaces (flex, studios, small office).

Are you building for pure investment purposes or are you looking to use some of the space for personal/personal business use?

I would also encourage you to look into the house hack space. 

Are there any specific clauses I should include in a room-by-room lease for a shared living space?

Then later as you grow older and no longer want to share your space you can just stop renting the other bedroom.

If you can find a place where the HOA or COA is flexible and you are able to convince the membership to go into development and perhaps convert the property into a co-op you could build a new apartment complex with commercial rental spaces on the ground floor and residential spaces on the top. 

That will allow you to build the additional legal space further amplifying the value of the proprety (more HELOC money). 

This amounts to lost opportunity because if you had purchased RE, at the closing it can start producing return. 5) ADUs detract from the existing structure whether this is privacy, a garage, or just yard space. 6) this is related to number 1, but there are many more buyers looking to purchase homes for their family than there are RE investors looking to purchase small unit count properties.