Would this contribution have to go into the expenses related to the purchase of the house, thus depreciated over 27.5 years, or would this be treated as a standard, deductible contribution, same as any other?

The duplex required new (AC, electrical panels, etc) I do want to utilize all the tax benefits and deductions from the expenses.

Deductible: This is not a coverage but rather your portion of a claim.

The HELOC monthly interest would currently be $1,300 per monthly (potentially tax deductible) versus the $1,800 net monthly cashflow from the 9 additional properties at $200 / month / each.

I was wondering if this is eligible as a business expense - or another type of qualifying deduction - so that I could finish up my income taxes. 

Hello,When using a HELOC to purchase another property is the interest not tax-deductible? 

Can I deduct those $10k on my 2020 Federal Tax return?  

I assume you mean income tax - I would suggest getting help from a CPA or a good bookkeeper, as you will have other deductions too - like depreciation, interest expenses, etc etc etc   - but for the mortgage, your interest is the expense. not the principal. 

Losses on the other hand can be currently harvested to benefit from the loss deduction.

Will the bill meet your deductible?