3. If I choose to add this functionality, what are the best (read: cheapest) ways to monitor humidity and CO2?
it could be said that quality costs money, what type of quality do you want ?if you use cheap sensors you might be able to get a system that works well, almost all the time.have you heard of the Boeing 777-MAX MCAS unit that relied on one sensor ?I would highly recomened that you do start with the simple stuff, but learn to read the data sheets.the humidity for mushrooms is near condensing and some data sheets show drop off of accuracy and life expectancy if run saturated or near saturated. opening the door to the unit can run humidity well past the dew point in the warm, moist environmental chambers you need.what you are asking is a great first project because 80% of what you want is well documented and easy to find the parts. there may be sites for arduin mushroom farmers so you might find riches there.as a note about misting. that takes pressure to atomize the droplets, again, cheap misters have almost no QC in manufacturing, and quality parts cost much-much more.to mist, you need a high pressure, and then the mistors.for high humidity you can use an ultrasonic humidifier. might be much easier to use.
Search the forum for "greenhouse" projects. Basic coding is not overly complicated, it's the hardware that deserves special considerations.For all external devices (sensors, humidity, light...) specialized driver modules are required. Many even unexpensive offers are available, it's up to you to find what you need.
This will be a trial run for feasibility before moving on up to larger and larger greenhouses.
Something you may not be aware of but last trip to Thailand NE I saw several mushroom growing "greenhouses" and not a drop of power or Arduino in sight, all natural heat, convection etc.etc.Are you trying to grow them in the snow somewhere?