Homebrewing - Beer, Mead, Wine, Cider

pasteurization then sweetening is your answer here if you don't want to use the standard potassium sorbate and potassium metabisulfate. Sometimes when I want to bottle carb something and then backsweeten it I'll just bottle carb it as per usual and serve it with simple syrup as well (if you fancy this is a lovely time to add fresh herbs too)

Your options to backsweeten are as follows;

1. Increase the alcohol content past the tolerance of the yeast. This will result in a beverage of 12% or higher ABV.

You can do this by starting with a higher initial gravity (this is my approach for sweet meads), or adding sugar until the fermentation stops restarting, or adding neutral spirits to bring the abv up, then backsweeten.

2. Ferment to dry, then add sulfites to prevent the yeast from reactivating, then backsweeten as desired.

Warning! This doesn't always work, and it's hard to predict how much sulfite is necessary, if you add too much it can negatively impact the flavor.

3. Ferment to dry, then use filters down to 1 micron to filter out the yeast, then backsweeten as desired.

4. Ferment to dry, then backsweeten just before consumption.

Most commercial producers filter and add sulfites.

Adding alcohol to bring your mead to 12-15% and then adding more sugar is the principle behind fortified wine like port. The chemical you’re referring to is sulphites, which are produced naturally by yeast anyway.

Has anyone tried using a very low attenuation yeast, like Lalbrew Windsor or Safale S-33?

The yeast has a tolerance to about ~12° ABV. Past that, it dies. So if you happen to sweeten your mead, it will not wake up since it has long gone to sleep.