I'm a little surprised you are still throwing a gooey sediment; this
wine smells like grapes, right? (No stink is what I mean). Concord
is grapey stuff.

I make sweet wine for other so identify with your situation. Tom S
hated sorbate and I usually come around to his camp; he never led me
wrong. Since you need to let this clear a bit anyway I am going to
try hot bottling and will post the results. The idea is to bottle the
wine 'warm' and let it cool slowly, the long contact time kills the
yeast. I have some extra mediocre Riesling I'm willing to experiment
with. I'll post the results.

It will be a little early for you to take advantage of if it works out
but you do have other options.

You can bottle it all dry, then take half a bottle and fill the rest
with sugar. Shake it until it dissolves, it will. (Yes, I'm
serious.) That syrup will keep for months in the fridge. She can
back-sweeten her wine to whatever level she likes and both of you are
happy. When she runs out of syrup, use the last half of the bottle
she opened to make more.

Sorbate can be problematic; it's commonly by home-winemakers used but
I think it's more trouble than it's worth.

Joe