[souffle]

I'm sorry for your losses.
In hammy years they were not really so young. Wet tail would only affect a younger hamster so it wasn't true wet tail they had. The most common cause of death in younger females is a womb infection or pyometra and these hams don't really display many symptoms till they are quite ill however some of the things you describe do suggest it may have been this. It could have been a stroke howver hamsters with a stroke mostly will survive only if they get through the first 24hrs and if there is no damage to crital organ function like the bladder and bowel. Antibiotics can give some remission from a pyo but it does come back soon. Pet store hamsters are usually from mass breeders and they are bred in numbers and often don't have the best start in life nor are the parents health screened so you do see lines that are prone to problems and may pass away younger than would be hoped. I doubt you would have any success with a complaint as they were of a reasonable age to be honest but you could contact customer services and mention what happened and that you would hope to have had them both much longer.
With your current baby it may be that she is now a much more settled hamster and is getting into a routine with her own timetable. Keep vigilant with checking her and ensure she is drinking. Leave food hidden and on the wheel to see it vanished so you know she is getting up. Try and teach her to come out when you rustle the shavings and keep handling. It's been cold lately too so make sure she has plenty tissue bedding and her room is comfortable.