Sorry to hear about your cat. We run a large boarding cattery (for 7 years) so may be able to give some answers; although we are not vets we have had lots of visits to vets.
Let me first deal with the vet seeming evasive - I once took a very ill boarder to the vets and during the consultation asked how long the cat could live. The vet replied "nobody can foretell the future".
If your vet had said your cat could only live six months you would have been devastated, likewise if he/she had said the cat could live for 10 years and it had passed away in a month (possibly from something completely different) you would still have been devastated.
Some may say the vet was covering his/her backside, and he probably was. The horrible truth is nobody can answer that question; we have had cats die aged 2 years or 5 years. They had organ problems that nobody could have known about.
Just continue giving your cat loving care, and enjoy all the time you have with her. When she does pass away, as she inevitably will someday, you will know that you have given her the best life possible, especially after her terrible start in life.
As for the leukemia - did the vet do a blood test to confirm it was feline leukaemia virus (FeLV)?
Our "bible" here at the cattery is "The FAB Boarding Cattery Manual" and this is what it says.
The bad news is that persistant infection usualy results in the death of the cat, HOWEVER, not all cats exposed to the FeLV become persistantly infected with the virus. Some cats can mount an effective immune response that eliminates the virus from the body after a short period of time.
It does say a lot more than that but I obviously can't type it all out. If there is any way of getting your email address to me I will be happy to scan the pages and send them to you.
Also try searching on "www.felipedia.org".
We do, in fact board cats with F