For example, if you purchase a $500k legal balance (unpaid principal + accrued interest/fees) for $200k, then rehab and sell the property for $300k, can you write off the uncollected $200k (the $500k - $300k difference i.e. not pay taxes on your profits, as on paper as the lender, it is a "loss")?

Ideally I'd like a HELOC for the flexibility and in an effort to avoid closing cost fees. 

You need to factor in  loan fees and such as well. 

There has to be a way of creating a network of low margin, high volume “ sale by agreement off network by owner” real estate transactions where the fee anticipates 2 transactions in a short time i.e the buy then the sell....

Be aware that if your family member decides not to pay you all or part of the agreed fee, you will have no legal recourse because only a licensed broker can collect a fee for brokering a real estate transaction. 

My original impression of rent to own was that it’s the same as regular rental, but with a buy date and additional funds being given as option fee.

Invoice them for damages, legal fees and get the property for the next tenant.

That includes (but not limited to) Mortgage payments, Taxes, Insurance, Utilities, HOA fees, etc. that occur during the Rehab phase and up until the property is fully rented.3. 

Someone is paying me a referral fee for just connecting them to a deal.

Plus, you'll have to see if there were any fees the brokerage house charged you that can be used to reduce the capital gains from the trading profits.As others who respond here will probably say, you should contact a good CPA who is very familiar with day traders and home office business expense.