PM2.5 have decent accuracy and are certainly useful at low price points, RH/temp work fine, particularly with separate sensors. Pressure works fine but is pretty useless. NDIR based CO2 (ex. Sensirion SCD31 or 41) also work fine.
TOC is a bit of a mystery, it can probably detect relative change. I've used them in a lab to monitor solvent vapor and you can detect changes, hard to quantify but don't need to most of the time.
I guess, yes and no. Technically the scd41 emits IR, it just doesn't detect transmitted light, instead sound produced by the CO2 that absorbed the light.
I can't seem to find a reasonably priced one that is actually accurate.