Those are humidifiers at best, the amount of CO2 they extract is negligible during the day and it goes negative during the night, they need to breathe too.
Yes, but technically, several hundred houseplants would be required to offset the indoor carbon dioxide to lower it sufficiently. This has been studied.
some people really love houseplants and go to long efforts to make them comfortable. although ive never heard of a plant hoarder, i can imagine a FD inspector raising eyebrows at a plant hoarding, esp. the fertilizers.