It’s coming up on the third week of secondary for my first mead batch and my intention was to sterilize the yeast culture with campden tablets and potassium sorbate. I added the correct dose of each for a 5.5 gallon batch after the SG reached 1.002. I assumed this would be dry enough that it would halt further fermentation, but 48 hours later the airlock is still bubbling at a period of about 18.3 seconds. Shining a light through the must shows fine bubbles rising to the top. I’m going to take another SG reading soon to see if there’s been any further changes, but do you think this would be safe to backsweeten without serious complications? The ABV is approximately 13.66% and the yeast I’m using (Lalvin 71-B) has an alcohol tolerance of 14%. Are there any further precautions I can take such that I don’t just send this thing into tertiary fermentation after racking?
CO2 bubbles doesn't necessarily mean that fermentation is ongoing. Bubbles are not all released at once. For example, with red wine kits, instructions often mention to stir for several minutes to release all bubbles before bottling. You can try this, use a sanitized spoon to stir and help release all CO2 faster.
Also the yeast alcohol tolerance is not a precise thing. Even if tolerance is 14%, it doesn't mean it cannot reach 15 or 16%, some yeasts cells will slow down but it is not all or nothing.
You better take additional SG readings, if the SG is stable for 3 days in a row, chances are that fermentation was stopped sucessfully. Then go ahead and backsweeten.