That plant doesn't look bad at all. Can you post pics of the leaves that turn leathery? It could be blight or some other tomato disease.

Basements naturally have more CO2, because it is heavier than air and falls. If you have a gas water heater or furnace down there, it will be making CO2 as well. Plus you and any other animal in your house exhales it with every breath.

Professional CO2 generators are simply a flame of natural gas or propane in a metal box. If you can find a portable propane heater that uses the small canisters of fuel, it's the same thing for about 1/10th of the price. You can use just the pilot light and it will burn a few days on one canister. Hang it over a plant to see if it makes a difference.

It is dangerous to allow exhaust from gas fired appliance in to the home. It contains a lot of harsh chemicals. I am a gas fitter for a living and have tested a lot of equipment and although the level of co is low if the appliance is fired correctly, it exists. There is a lot of heavy metals in gas after burning...
I wanted to figure out a way to charge a co2 scrubber and then release the content, for fun...

I have been using co2 blocks to add co2 in to the grow area and I am having success with the leaf curl. I think the problem was that the plants where growing so fast they ran out of co2 and not able to grow right. I am also opening the door at night to cool the crop and closing when the light is on...