If fixtures are not used the p-traps will not carry any water allowing sewer gas to pass through.

I've found that in a situation like this the homeowner is more interested in doing an owner carry on the mortgage.

…but I feel like every minute counts in minimizing those HML carrying costs, unloading the flip and moving on, especially since the rehab itself took about a month & a half over the initial timeframe (due to no fault of the contractor’s).

Is so, you've a 99% chance it is load bearing.That does not mean you cannot some or most of that wall, but you will have to put a beam across that span (LVL or steel) and ensure the load is carried all the way down to footings under the house.  

We are being very conservative in our underwriting as I would prefer to remain static with little to no vacancy (<2% forecast for the coming year based on present leases) than add 50 units which we might b struggling to carry when the market moves to a further increase in vacancy.

Regarding point 1), you might consider owner financing to a buyer and carrying the note as a passive income source. 

Don't forget that it's highly recommended that you carry E&O insurance as well.

The last time I was looking for a bank that would be a portfolio lender that would allow seller carry back financing for part of the deal it took me over 20 banks for me to find one who would lend. 

You don't want to do any long term installment contract without an attorney, if you can find one to do one.The DOS and FHA regulations are anchored in federal law, not administrative "discipline" and administrative rulings carry the weight of federal law. 

The lease and the option are two separate agreements carried out simultaneously.