I've been using Guaranteed Rate for my last 2 buys, never used them for a cash-out refi but my lender is not confident that I can get 75%-80% LTV for that, but instead 70%.

We're confident we should be able to have the appraisal adjusted to at least the 325k range.

Your dad's credit score will increase. 

The time may increase on Inspections but a Stop work order ,for not filing a building permit ,will be greater.

Can the rates increase at a dangerous rate since they aren't fixed?

I know this is a business to make money, but if they were to move out or you had to evict because they can't afford the increased electric bill and the rent, then you'd be out even more money with vacancy and turnover costs.

We've increased rents on 2 of the other 3 units and soon to be all 3 of the other units.

However, I have not used co-signers before and was curious in what cases other experienced landlords would allow this.So basically the question is: do you use a co-signer as a replacement for bad credit, or only as an extra safety net in cases where the credit is marginal and you aren’t 100% confident in the tenants ability to pay rent on their own but you “think” they “should” be able to?

This can affect the feasibility of collateralizing your hard money loan, and the refi (you might need two, or three, loans instead of one, which not only increases the difficulty of getting the refi(s) through underwriting, but could mean 2-3 sets of origination and closing costs).

If your happy with them and feel confident thy can get the deal done.