Ohhhh, I'm really sorry to hear that. You've put so much work into Mille.
Consider adopting the cozy and tame cat before giving Mille back. Sometimes a shy/feral cat can learn trusting behavior better from a socialized domestic cat much faster than they learn from us humans. The first few weeks I had Farrell, she definitely watched very closely as my domestic cat Tangent rubbed all over me and flopped over for belly rubs. I think Tangent helped tame Farrell more than I did.
Consider adopting the cozy and tame cat before giving Mille back. Sometimes a shy/feral cat can learn trusting behavior better from a socialized domestic cat much faster than they learn from us humans. The first few weeks I had Farrell, she definitely watched very closely as my domestic cat Tangent rubbed all over me and flopped over for belly rubs. I think Tangent helped tame Farrell more than I did.