It is what reactors you might have on hand that you could connect together to obtain a high conversion.
One can't use the definition of total conversion up to a point because the reactant is fed to the stream between reactors under these circumstances. One must work in terms of molar flow rates when writing the mole balances.
Yes, for some adiabatic, exothermic reactions.
You would if you had a PFR large enough. We are assuming that you have these reactors available for your use. See the ICM-Staging.
Yes.