I wouldn’t trust ARVs since they are not set in stone, and can change with market fluctuations.

There are a few more loan products you must chose from, planning requires a great deal of care to avoid excessive holding cost (how long will permitting take, build timeline, labor and material price fluctuations, how many other projects is your builder involved with, current shortages in supply chains, etc.).

Most uncertain or fluctuating price items as far as entire development goes are Land and Sale prices, but other areas remain fairly possible to estimate at very early stage in project.Even then, Why does following areas are considered some of the challenges for novice to seasoned developers despite having so many tools nowadays to help them.A] Estimating fairly accurate Construction Costs(Hard + Soft)B] Controlling Costs, BudgetC] Financial structure (equity, debt, draws, taxation, insurance)Thanking in advance

Also since market is fluctuating and we are newbies.  

I didn't know the rate fluctuations so much.

But, in my opinion, with the 3 "slushfunds" accumulating money, and not counting that money as part of your return (or pocketing it, for that matter), your annual ROI wont fluctuate as much, as it will always go against your initiatl capital investment of $15k. 

Cons of owning/ house hacking is running a business if you don't have systems or professionals in place (property management, repairs, etc), market fluctuations can change rental and exit strategies. 

This is a MASSIVE generalization because price will fluctuate based on needs of the home, style of the HVAC (split, package, wall package, mini-split), filtration in the home, etc.

Any current experiment or previous experience with vacation rentals and having fluctuating times throughout the year of being rented out?

Both parties are aware, or should be aware, that real estate markets are fluid and prices fluctuate, creating the possibility the value at the time the option is exercised may be higher or lower.