1) Relay/SSR for lighting control -or- a photosensor to detect lighting... elevated CO2 is wasted if there's no metabolism occurring.. with this much effort, you might as well maximize the efficiency by handling the lighting integration. The "dead band" logic you are talking about is referred to as "PID" in some circles, that type of programming is an art unto itself.. there's a well-done PID library for Arduino out there, you might want to look at it, if even to see how someone else came at the problem of using digital means to control analog processes...
2) Serial output of the collected data.. keep a rolling buffer of data (you should be able to make the data pretty small with a little creative use of int/char) that you can connect to the unit and download the buffered data. Make the data comma-delimited, so you can import it directly into spreadsheets. A query/response via serial is easy enough, and this will save you pencils and grief. Alternatively, you could add an SD card and log the data to the card... which is probably the better option. You could store months of data on an SD card, even if taken at pretty tight intervals.
3) Recently read an article (may have been Nature or SciAm) on CO2 and hydroculture, and a recent trend of using CO2 for pest control. The idea is that periodically, say every few days, the CO2 levels are taken VERY high for a few hours during night.. in the range of five to ten times the PPM you normally "enhance" the growing air to. Apparently the idea is to suffocate insects and eggs, and the level is apparently even enough to kill many fungi and things, as that kind of level throws a pH monkeywrench into almost anything's respiration... but green plants take it okay as long as it's only for a few hours. Neat idea, it has to be better than dangerous chemical insecticides, I would think. It seems such a great idea, maybe you can give it a try and let us know if it works!
Cute device, great job!
As an aside, in the same article, it spoke on using yeast as the generator for the CO2.. brewer's yeast and sugar is apparently a great way to "naturally" produce CO2, less costly and more environmentally friendly than cylinders. I would think that using yeast you'd have little danger of raising levels too high; though I suppose the only "regulator" would be a feed system for the sugar.. but that would take hours at least, I would think, to change the air CO2 concentration.. Just a thought, anyway
If I can find the article I'll post a link. Interesting read, was a really good discussion of the use of CO2 in greenhouse hydroculture and it's ups and downs. I'm sure you already know this (but I'll say it for the kids reading this), but please be careful.. Carbon Dioxide displaces "Normal" air and it's possible to actually black out and then subsequently perish if you make too big a mistake and walk into a room at some ridiculous level of CO2 concentration. I'd hate to hear a software glitch led to some tinkerer's untimely demise!