Use extended closing periods or additional earnest to buy more time for your old property to sell if you need.The last option is what's called a reverse exchange.

Another option is to co-sign but that has the same impact on my credit and responsibility to the bank.  

@Pat Aboukhaled I would ULS (unit lot segment) the primary house and include a clause in the call option that would prevent them acquiring the DADU.

I’ve done a couple of fix-and-flip projects in the past using a hard money lender (HML), which has its pros and cons.I’m currently looking at a couple more properties and trying to decide if I should go with an HML again or explore other financing options.

It works great for many people -- not me -- but it might be your best option considering your distance. 

Your best and most efficient option would be a DSCR cashout refinance, more than likely.

Another option would be to sell, reinvest into a house hack and use the excess on a deal.

I would suggest that you find a partner or mento who has done it before.Many of the investors that I work with in your position find that buying New Construction investment property from a builder is a better option. 

would love to see a seller finance specific calculator that has a option for 1031 exchange from buyer bringing in down payment.  

Have you and your wife considered exploring fully turnkey properties as an option?