Getting her spayed will stop her screaming and may make her happier. The Ministry vet will not do this while she is actually in heat though. Rather than putting her out into the cruel world, if you don't think she is rehomable, you could have her put to sleep. It sounds horrible but it is kinder than leaving her to fend for herself. She has already had more love and care than the other feral cats. Bless you for caring so much. I hope it works out for you. JJ