I'm not an expert...so...
1. Seems like a lot of organic material in the substrate breaking down. Not sure of the chemical processes there, but it seems risky. As far as I know, vermiculite is inert and used in terrestrial to prevent soil compaction and improve aeration. With the granular aggregates we typically use in aquatics those trait are already present. I think the vermiculite is redundant.
2. I'm sure there is a limit, but it seems unlikely you would reach it using normal injection methods.
3. Most do. Plants "inhale" oxygen in the dark.
4. Sure. Activated carbon or Purigen
5. Not for topping off, but is preferable during water changes. People seem to like Equilibrium.
6. You'll need a test kit. Most local places have them now.
7. Sky is the limit. You can test and moniter virtually every physical and chemical property in the tank. pH, GH, KH, Fe, N, P, & K are the most common for plants. Test kits are available for all. Some can be done electronically. Check LaMotte. There are also a host of minor nutrients, but I'm not familiar with common testing methods for them.
8. Not sure.
9. A lot of people seem to use the Seachem Flourish line. There are several others, too. Most around here like buying dry bulk chemicals and making custom mixes. Check "PMDD" and "Barr EI"