Please note: .. SO2 "WILL NOT" arrest a primary fermentation!!!
Cold temp will slow it down .... VERY cold temp will bring it to a
virtual stop! But, you need to "separate" the yeast from the wine to
yield a sweet product, or else "off it goes" after the temp comes back
up. Nearly all wine yeast species are SO2 "tolerant" to a great
degree! Sorbate is a "brute force" method of stabilization. Sorbate
prevents yeast reproduction. You must use an external method
(chilling) to separate the wine from the yeast, in order for sorbate to
be very effective, and even if it does stabilize the wine's sweetness,
it contributes an undesireable flavor.

Long story short - making sweet wine is a bitch! It's been said many
times here .. for small batches, make it dry - add the sugar before
serving!! Otherwise ... get a REAL sterile filter ($$$) and learn to
use it!!!

As for SO2 amt ... 20 ppm free would give you 0.8ppm molecular at pH
3.2. At pH 3.6 you need 50ppm free to get the desired 0.8ppm
molecular . However, even the low level of SO2 you have would arrest
ML according to my experiece.

Charlie