From your description, you are very lucky your vet could save this one - I think she's entitled to be a bit slow after going so close to death!
Sometimes they seem to never look back after being treated, sometimes they still seem 'fragile' so I might bring them nearer to home, give them a shelter to use, etc - mollycoddle them a bit, and watch for another bout.
I had one, one year, collapsed three times over about 4 weeks, but eventually produced two strapping twins and never looked back thereafter.
BTW, you should always warm calcium / magnesium to blood heat before injecting, whether sub-cut or i-v.