When you want to backsweeten a wine/mead/cider, it's a 3-step process:
1. Age long enough (months-year),or cold crash and use fining to eliminate all visible yeast. In other words, get it so clear you can read through it. The yeast won't all be gone, but numbers will be low.
2. Dose with both potassium metabisulphite (Camden) and potassium sorbate.
Wait a day or two and backsweeten.
3. Give her another week to make sure fermentation hasn't restarted before bottling.
As an alternative, you can fine filter, or pasteurize. I pasteurize my Grafs, which I want slightly sweet and moderately bubbly, but I don't anything else. And I only sulfite and sorbate the meads/wines that I'm going to backsweeten. Even then, I've occasionally had a cork pop out, especially early on in my wine/mead career.