Fred, the fact that you've had only 5 corks blow in 7 months indicates the fermentation rate is very low. If the ferment is slow enough, the sorbate will inhibit the yeast, and fermentation won't progress, which is why I suggested add it now. If the SG doesn't change during the following month, you're set and can re-bottle. If it does progress, you're no farther behind.
I agree with @VinesnBines
that your wine may not have been as homogeneous as you believed. Over the past year I noticed that I did not mix kits as well as I thought I did -- I consistently got a different SG reading the following morning before I inoculated batches. The differences are not always significant (+/- 0.002), but some were. This was a bit of a shock to me, as I was sure
I did a good job of mixing, but my results indicate I did not.
Conditioner and glycerin are both very thick, and do not distribute easily. Another point may be the age of the yeast -- if the wine was a few months old (I can't recall what you said), the yeast was older, and it's not necessarily evenly distributed. So it's likely you had pockets of viable yeast and pockets of non-viable yeast.
My current method for glycerin is to start the final siphon, add K-meta (and sorbate for the few wines I backsweeten), and direct the siphon into the measuring cup that holds the glycerin. I use the stream to flush out and rinse the cup, then when I have a couple inches of wine in the fermenter, I stir manually for 30 second. Half way through and at the end I stir again, this time with a drill-mounted rod, about 1 minute each time, changing direction half way through.
I have FWK Strawberry and Blackberry in production -- each kit received a conditioner pack + bottling pack (K-meta + sorbate) before going into bulk aging. Each has another conditioner pack, which I will add prior to bottling in 4 to 6 weeks, using the glycerin process.
I'm honestly surprised, but these kits probably need both. At bottling time I'll check SG to compare against the pre-bulk age value, and again after the final conditioner addition. The SG of the dry Strawberry was 0.996 and post-conditioner was 1.006. Based upon that, if the SG still measures 1.006, if I add the full second pack, I should get ~1.016. If the current SG is not 1.006, it will indicate I need to improve my mixing.