Feral cats that have since become indoors should stay indoors.

one independant girl decided to take off one day and she came back 3 hours later with a rather bizare eye injury.
I think you just answered your own question. Formerly feral cats CAN become indoor cats. I have a formerly feral kitty, Lil Eli. He's perfectly happy being indoors only.

I hope you make the right decision. Think of your cats health and well-being.