I know anecdotes don’t trump science but I just wanted to add that I am using the exact same vials and the same ratios and have successfully restored a number of yeasts after several years. I just spun up a danish lager yeast that was 3.5 years old and it tastes and smells fine. I don’t have equipment to look at it any closer or a more scientific method than smell and taste unfortunately...Always keen to learn, I shall read that Mat when I get the chance. (Retirement does not make me less time poor in lockdown)
Actually in my method above, Mark, I am using 25% Glycerine, 25% water and 50% yeast slurry i.e. 12.5ml glycerine 12.5ml water and 25ml yeast slurry.
These solutions do indeed freeze solid and so far have had no failures. Bear in mind I do use a decent overpitch of about 20% in the brew (backyard calcs)