Kevin,
Without being argumentative, the initial post doesn't mention a cleaning product only a specific sealer. I apologise if I've assumed the cleaning product to be one of yours. I was more interested in the rational behind the determination of the fault.
Victorian tiles are porous and laid on a porous substrate, or at least porous enough to want to absorb water and chemicals at a rate unsuitable for dwelling with a chemical. Such practices as pre-wetting and dwelling of chemicals are just asking for problems. We've even seen similar problems on these floors when one guy has worked alone, by scrubbing, stopping and vacuuming up. Be it too much water or chemical the after effect is very similar.
Why does the salt take days to appear? Because the alkaline ingredients of the cleaner more dense than the readily evaporated water used for dilution, rinsing, pre-wetting. As the water evaporates up through the tile body, salts deposits are left behind, naturally these deposits are going to be minute, they'd have to be to pass through the pores of a tile, but they accumulate.
The application of a sealer no matter how good it is, will impede this moisture transfer at some level. Somewhat akin to putting klingfilm over a plant, the moisture will escape but slowly, leaving behind a moisture residue.
Personally I wouldn't advise leaving a customer with an unsealed floor for any longer than absolutely necessary, can they be trusted? Look how they treat their floors before we restore them
