I had initially planned to finance the rehab with cash and creative financing (lines of credit and such), and then financing it all through a bank.

I must admit, I am struggling a little on this one.We already made an offer on the home and the counter was decent, still keeping us below initial asking price.The offer was with indication that we would be doing an independent home inspection, always a smart idea.At first visit to property, there was concern on the diagonal/zig zagging lines in the walls, the few gaping holes where the stairs meet the home – evidence that the staircases had at one point pulled away/separated from the home.This cracking is by no means just settling, my limited knowledge of home construction was telling me, this here was a structural issue.But wait….. there’s more!

Is it ok for me to use a loan or low-interest credit line of my own to pay the mortgage on the duplex before I get renters in it?

This is in line with their goal to protect the tenant.

I'd like to be 1st in line for 2nd ownership.

First time my place was trashed by some festival goers (puke and urin on beds, beer everywhere) and my maid cleaned up and disposed of the trash before airbnb told me I needed to take photos. 

While out driving today I found a 1083sf 3 bed 1 bath home for sale $8,900 or best offer.  

I don’t have cash but I know I can get approved for a line of credit that would cover my down payment.

You'll have 24/7 access to the house (you'll be lining in it) and you can see how others are treating your property.

But if you have a hard time getting a portfolio loan then you might try getting a business loan or a business line of credit that is secured by the equity of those properties.