On the first point the assumption is normally made that when someone buys a Ph controller with their kit that they have gone for the works. Highlight et al. In a highligh tank where CO2 stability is much more necessary then there will be swings of a up to a point during the day.
With the drop checker I am by no means someone who acclaims the CO2 indicator. I am however one who uses two for the reason of being able to see the levels 'cheaply'.
I can't understand why yours starts at yellow and then slowly comes down through to blue. It should always start at blue with 4dKH and bromo blue. The reason we use 4dKH is because it is a known reference and after 2-3 weeks we empty rinse and refill in case of contaminants.
It should change colour over several hours rather than days. If so then something is not right with or within the DC.
I don't think anyone can say the system is flawed as it has been used for a fair few years succesfully now and was never supposed to be highly accurate, hence being called an indicator.
As for the other points I am no scientist so someone else will have to give 'real' responses to these but:
I don't see any reason why other gaseous substances can't pass through into the reagent. What works for one should work for all here I suppose. I would say though that Hydrogen Sulphide should be in such low concentrations within a tank otherwise there is a much bigger problem that you need to worry about than the drop checker. I guess small quantities could change the Ph though and therefore this point is for someone else to answer
I remember something about Brownian Motion in schol and though it was something to do with heat. With the heat within the drop checker and tank water all being equalised after a short while I would've though that this wouldn't occur. Is Brownian motion the one where hot water rises and cold water sinks? If so then you have solved the nutrient distribution due to poor flow problem as we can now just heat the lower half of the tank and chill the top half and let brownian motion distribute the nutrient for us
Again I am no scientist and this is a question for someone else. Is brownian motion the smoke one? I can't remember science was never one of my faves.
As for the final part. Yes once someone has the feel for it they could in theory abandon both the controller and the DC but why? I never had a controller anyway and a DC costs so little to run that if working properly (changing by the hour) then it is surely a cheap safety feature that is pointless to abandon. Just for you to see here is a timeline of mine showing how quickly (at longest) they should change from fresh solution (bright blue 11am) to lime green (9pm):
Hope this helps a little.
AC