Although I don't want to be actively working on more than one home at a time, I would like to reduce the downtime between completing and listing one house and starting on the next.

, make sure you are focusing on the right things to do this (BUSINESS).One way to do this is to:Get enough cash together (saved in an account) to buy what you want.Also  have enough cash to pay any closing costs, and repairs needed to make the place rentable.Have enough for anticipated cash reserves (learn about this--it's important).Hook up with a good property manager (before you buy and get some knowledge there).Find a place you might want to buy--walk it in person, have a home inspector walk it and give you  written report.Walk it in person with a general contractor (pay him to do this), who can give you a bid on the repairs--use that contractor if the bids seem fair.Make an offer that allows you a nice profit on the rents.If accepted, do repairs and have your property manager put a renter in there for you.Know the cost of a turn, and an eviction, including the down time when you will not get rent money.

Cash Flow is protection in down times and you should not buy for only appreciation.

That means that all the down time with no tenants isn't your propblem.

I’ve had down time, so I want to maximize it before I ramp up for the next deal.

Plus you need the reserve cash fund to cover expenses and downtime. 

There is very little down time or fun time.

Downtime is a killer because if we're not working, we're not getting paid.And for the record, I'm not always opposed to homeowners doing their own demo.

I was hoping I would be able to put in extra hours after work and during down-times with my full-time job, so that I would be able to make an extra 5-10K which, in turn, would help me save quicker towards my first down-payment on a house hack.

That said, if she really insists on not moving in then market it immediately to start mitigating your down time while also informing her in writing of her responsibility.