This particular property is intended for the couple to live in it probably on a permanent basis. 

The fees incurred to do this refinance are apparently much lower than a permanent mortgage.

I don't think I would turn it into a rental when I was done, I imagine that I would sell it.My goal with real estate is to invest in a stable and permanent place to live.

I am aiming to buy $1.5m to $2m apartment complex soon in the near future but I'm not even sure if I'm qualified for permanent financing yet?

If you are seeking to change the behavior of an RV park from transient to permanent, consider installing "Park Model RVs" which if you are not familiar with, are smaller versions of a mobile home that comply with the zoning regulations for RV parks. 

The objective is to fix up the foreclosure property and flip it (or rent) and also use it as a base to develop my current piece of property to become my permanent home.The foreclosure estimated price is about $450,000. 

I bet if you keep pushing with local banks, especially those popular within your local investing community, you'll find one that does HELOCs on investment properties.If not, or perhaps even if so, you could always do a Cash-out Refinance with a lender and have a permanently amortized loan that get's you the cash.  

My main goal is to end this issue permanently so I don't have to deal with this headache.

Doing that will (as you may already know) will avoid PMI when you refi into a permanent loan, thus satisfying your HELOC.

Fine.There was concern that covid would be so widespread and damaging as to cripple the economy permanently.