This includes depreciation calculations.Of the 1/3 of expenses that are left, only some are then deductible, but are then reported on your Schedule A.  

I have my would-be tenants pay for the screening and I offer to deduct that amount from the first month's rent.

Why wouldn't you take the 7.5k that you currently have, pay it against the 15k lien to parents, keep the mortgage and LOC in place as is, and re do the amortization on the15k lien (now 7.5k) so you will cash flow positive from day 1, take advantage of full tax deductions, enjoy the appreciation and avoid unnecessary commissions to 3rd parties?

With the strategy above, there is no paper trail, and they can deduct that you own nothing of value, or that you appear savvy enough to make them go after easier targets instead of you.

Car loans are not deductible.

We also give the option to purchase a home warranty with anywhere from a $45 to a $100 deductible and we write the lease up where the tenant pays the first $45 to $100 for repairs that were not due to normal wear and tear.  

My biggest question is won't I lose the tax benefits of deducting loan interest if I use a private money lender and exactly what does that look like from a business standpoint?

Effectively, your reserving CapEx cash is funding your Depreciation Deduction. 

Here it is:1 - Must be able to get at least enough cash back out of the deal within 6 months to move into the next deal.2 - Must get all of my cash back that I put in within the first year (this is cash...not equity, tax deductions, etc...)3 - Items #1 & 2 must be satisfied while we cash flow at least $450/month with all expenses figured in...including a PM.

After paying the $55 deductible, their contractor refused to make the repair.