You just need to clear it with the underlying lender.

The more complex you make your LLC situation (particularly with unseasoned proxy LLCs) the less appealing they will be to a lender. 

Are there mortgages on the properties?

Most mortgages are fixed. 2) value adds.

I had question about the mortgage interest deduction on my primary home.

An assumable mortgage is effectively a 'portable' loan.

The home is worth $1.3mm today, and I have $700k remaining mortgage and $600k equity in the house.

Hey Bob,Depending on the lender, I am assuming you are working with some hard money lenders, their appraisal or BPO will probably include an opinion subject to the scope of work.ARV is relatively easy to figure out yourself, made easier and more accurate working with trusted agents and managers.Im not as active as I should be but I have seen several Facebook groups dedicated to Florida Multifamily deals.

Hypothetically, if I took over someones existing mortgage subject to and owned the property cause I have the title now, would I be able to get a home equity line of credit or equity loan on that property if it had a good amount of equity?

It got me thinking about new construction for single family with an ADU or multi-family, and house hack to offset the mortgage.