ok i don't know a lot of your ques but i do know of a good substrate its called flormax but you will have to move the betta so it can cycle all over again or not move it and risk death
Thank you for your recommendations! This is pretty much exactly what I was looking forThe only issue I can see with natural lighting is just how strong it is compared to artificial lighting. Even indirect natural sunlight can cause issues with algae. I have two tanks on the top shelf of my rack and the only light they receive is whatever natural light comes through the clerestory window. The plants in both tanks are thriving, but I also get a whole lot of algae along the back glass.
Also, in a 1 gallon tank you'd want to probably make sure that the sunlight wasn't causing any fluctuations in temperature as that is not a lot of water to heat up.
For the plants you are considering, I would not think CO2 is necessary. In my planted tanks, I personally like to use a nutrient rich substrate such as aqua soil. Then if necessary, you can supplement with fertilisers. That's been enough for me to grow plants like watersprite, hydrilla (similar to elodea/anacharis), hornwort and duckweed without much of an issue.
Heavily planting a 1 gallon tank can drastically reduce the amount of water changes needed depending on the plants you use. Fast growing species can often take over the role occupied by a filter in a cycled tank, by uptaking ammonia and nitrates from the water. I personally recommend people with smaller tanks utilise live plants as much as possible as it does go a long way towards improving water quality and keeping parameters stable.