Winemakers typically add sorbate (aka sorbic acid, often purchased as potassium sorbate) when they want to bottle a wine with a little residual sugar. It is often added right before backsweetening and bottling. Sorbate will inhibit the reproduction of yeast cells but it will not “kill” yeast, nor will it inhibit or kill bacteria. It should always be added in the recommended doses; too short of a dose will not inhibit the yeast from continued growth.
As you hint in your letter, if you add sorbate before the ML fermentation is complete, you run the risk of a geranium-like off-odor that can really ruin the aromas and flavors of your wine. For this reason, I recommend that you wait until your “malic acid spot” on your chromatograph disappears entirely, which means that you’ll have an ML complete wine.
It’s really difficult to determine the amount of lactic acid you have based on the size of a spot on a chromatograph, so I always have just gone with the “malic spot” disappearing. Because chromatography is tricky to do and somewhat inexact, I’ve always been a fan of sending in my wine for malate analysis when you think it might be done (often when you hear the little CO2 bubbles stop ‘ticking’ in the fermenter).