They can dictate things like minimum coverages and deductibles.

Generally, that comes with a higher price tag, even if everything else is simpler, they don't discount their hourly rates just because some of your work is easier for them.Sometimes the value proposition is not that they found some missing deduction/credit or election, often it is small stuff, plus having the knowledge to keep you out of trouble.

Deferring deductions.

It can also incorporate added items or deducted items along with the costs associated with them.

Can I deduct all of these costs when against my proceeds when I do my taxes for a conventional sale? 

Both are tax deductible but you are still out of that money.

I am confident I can capitalize the renovation/construction/zoning costs, but what about property taxes paid in 2018 (all of which exceeded the $10,000 SALT cap based on our W-2 state taxes, so none have been deducted). 

But you do not deduct it until the year of sale.

If I pull 100K from my house and then leverage that x 80%, then I get a total position of $180,000 and its only costing me 4.71% interest per year (fully tax deductible!).

I would trust them to do this as they have already been handling all the small maintenance themselves and organizing professional repairs when necessary, which we deduct from rent.