I'm new to this forum but I can confirm that the Syringa afghanica has a very strong hardiness: it lives very well in very cold conditions.
I've planted last october a joung plant in my alpine garten in Valle di Casies (1200 m in North East Italy): this winter was long and very cold (-20, -25 ° C)
The first frost in November, snow (40 - 50 cm) till March-April, a very cold springstime and the soil iced till the middle of April.