very well explained and yet concise answer, of benefit to many, no doubt!!Can't give you an answer but can give you points to consider.
If you simply warmed the water to circa 35-40'C to dissolve then naturally occuring Yeast & Mould in sugar will survive and germinate. If you took the water to 85'C, dissolved and then cooled you have provided an effective loading reduction (including from the pan and utensils etc). The general Y&M 4 log reduction rule is 85'C/5 mins, but what with cooling you are fine.
Next consider cleanliness of storage vessel and exposure to Y&M when decanting.
I go to 85 and decant hot, effectively letting the syrup sterilise the storage container if I know I need to store it and get 2/3 weeks (at ambient) unopened but it makes diddly squat diff once in the feeder!
Adding HBH to the syrup when you make it will resolve a lot of the fermentation/mould issues. It also is very useful in sprays as the smell tends to mask some of the bee phermones and can make them somewhat calmer.i made up some 1:1 syrup about a week and a half ago, will it still be ok, i want to use it in a spray!