Some do use it as a pre spray but presprays have additives that aid cleaning but cannot be left on a carpet which is not a problem as they are rinsed off, F90 does not have the benefit of these additives as a certain amount of it will be left in the carpet.... You might as well use a dedicated pre-spray.
I'm not that familier with chemical injection system so can't advice of their usage but I'm sure the instructions on the containers gives the details.
Yep it would leave the carpet alkaline while damp, dry carpet has no ph reading