Restarted fermenation problem

I have a problem.

I started a 1 gallon batch of watermellon/strawberry wine on 8/25.
It had a starting SG 1.100 and a final SG of 0.992. Its on the strong side
15+% alcohol. I used Lalvin L-1118 yeast. It fermenated fast.
I racked it and added 1/2TSP of potassium sorbate and 1 crushed campden tablet to stabilize and backed sweeted it to 1.012 on 8/31.
On 9/3 I noticed a couple a couple bubbles today I have full fermenation. What went wrong and how do I fix it? Should I add another campden tablets and more potassium sorbate? If I don't do something it will end up 18% which is L-1118 limit?
Also, I used Baker and Crosby campden tablets with have 30 ppm free SO2. I was reading Carlons campdens tablets have 150 ppm. Is this correct? Should I use 1 or 2 Crosby and Baker? This should be standardized.

I guess the sorbate and campden didn't do the trick.

I'd let it ferment out, and then try again. The key to sorbate and campden is the racking. Sorbate doesn't kill yeast, but keeps it from reproducing. In order for it to work, you should have a clear wine (less yeast in suspension) that is completely finished. If you started it on 8/25, you're way too early to consider it done. That was the mistake here, I believe.

It's going to have a higher ABV than you planned, but it may not get up to 18% unless the yeast is very happy. When it was finished the first time, 1.100 to .992, that's about 14.2% ABV. If you sweetened to 1.012, and it ferments fully, you'll be adding another 2.6%. So, not 18% but over 16%!

I'd let it finish, or chill it. Chilling it will drop out some of the yeast and slow fermentation. Then rack it to get it off of the yeast. Once it's clear and been racked (either after fermentation is finished or through chilling), you can keep it in the carboy for a while to make sure it's finished and not dropping any more lees.

Once you're certain that there is very little yeast still in the wine, you can try using sorbate and campden again. I'm not sure of the "dosage" of the brand you mention, since I've never heard of it, but 50 ppm of free SO2 is about right. For the sorbate, 2/3 teaspoon per gallon is good. Make sure the sorbate is fresh, and not older. Dissolve the sorbate and campden into some boiling water (1/4 cup is plenty) and then rack the wine into it. Wait three days or longer, and then sweeten to taste.

What about using a non-fermentable sweetener instead? (Obviously for "next time," of course; the genie is already outta the bottle for this one. )