But if it's a refi from one conventional loan to another, I'd probably be more inclined to pay the fees up-front and lock in the interest rate while it's at a historically low level, and just avoid the refi altogether.

Normally, with Mexican banks, it's 50% and the interest rate is in the double digits!

There are local credit unions out there who are doing low and no money down conventional loan products with no pmi, you just need to shop around.  

I intend to finance the purchase with a conventional 30y 20% down loan.

I currently own my home as a conventional loan as it was a rental, so I needed to put down 20%.

FYI, most conventional lenders (99% of all mortgages being done these days) will not fund deals from wholesalers.

Why not use conventional financing? 

The same way you qualify for any conventional loan. 

Can a primary residence be converted into a rental property after a refi since it was a conventional and not a investment loan?

However, once you do that, and your credit score improves, you can then refi back to Conventional/jumbo loans.So it's an annoying 2-step process, which incurs more cost, but just bite the bullet and do it.