I'm planning to apply for mortgage for the fourth home as primary residence(CA), Will I be able to have mortgage interest deductible since it will be my primary residence?

Keep in mind that "gifts" are only deductible up to $25 per person per year (26 USC 274(b).).

With an S-Corp, you can run into basis limitation issues that would prevent you from deducting losses from your rental activity (if you are able to take those deductions) - just one of the many reasons not to own rental property in an S-Corp. 

I adjusted the tabular values below to reflect a tax-deductible/pretax contribution, pushing the total outlay higher on this analysis. 

How much of a gross sale would you have to make to earn that much commission after all deductions and sharing commissions (not to mention all the moving parts of that deal that you'd be involved with.....)?

Scripts are a set of standard answers to many of the common questions people ask.

So here is how it went: The listing had expired, so I contacted the bank directly as a Realtor and the "standard" commission at the time was 7%We put in offer on the house for $22,000We were going to go for a 95% PMI Conventional Mortgage, which of course meant we needed 5% Dn Payment.I used the commission agreement as my down payment, and got a check back at the closing for 2%.We moved into the property as we rehabbed it, which took about 3 years (we were actually not in a hurry to sell it).

Even with her limited income I would think you could still be an accommodator on her mortgage if you wanted to pull more money out or just go 1% ownership to be able to deduct interest.

This is your standard practice that applies to everybody." 

You are at a little higher risk and liability because you are licensed therefore you are the professional and held to a higher standard.