The tenant frequently informs me the interior temperature only reaches 68 degrees.

Quote from @Paul Gupta: Hi, in California, For jobs requiring a license, there was CSLB (contractor state licensing board) which would give either a General Contractor license, or specialty licenses by trade (~60 specialties such as concrete, welding tile, ) https://www.cslb.ca.gov/about_us/library/licensing_classific...My Q: does Maryland and DC area have similar requirements? 

Where I am the current temperature is about 10 F right now.

If the temperature goes below freezing point, pipes might burst. 

If the temperature goes below freezing point, pipes might burst.

The problem ended up being so severe the pest company had to tent the house and raise its temperature to something like 110 degrees for a few days.

just don't do a lease. as others have mentioned, this forces the buyer of the property to take over the lease (if its even allowed/ they qualify) and it certainly does make the property a little tougher to sell. i'm also doing a house hack, and here in CA, its made a world of difference for me. the bulk of our energy usage is in the summer when we want to run the a/c. my bill stays a constant $228/mo (i pocketed the tax credit and simply financed the panels after that grace period). instead of getting an $800 bill in the summer months, we run the a/c at a constant temperature 24/7 on auto, stay comfortable, and i typically get a little bit coming back at the end of the year. don't expect to pocket this "little bit back" by being conservative with your useage, it sort of is a use-it-or-lose-it situation... i think i got $54 back this year.

For dining chairs, web restaurant store has some really nice fully welded metal chairs that are a single piece aside from the seat and back panels.

You can probably find an online link about how extreme temperature changes are not the ideal way to heat a home and cost more in the long run.I don't know what the local rental rules are where you are, but look them up and see if you can give her notice. 

@Jeremy Bloom  the rag on the thermostat can be beat by placing a remote thermostat that is set to the legally required temperature with that remote thermostat actually controlling the temperature.