We never explained to Mum that she would be staying permanently - it would have caused her a great deal of distress/ anger, and she would have forgotten it within minutes anyway.
Far kinder, though less honest I know, was to say that the doctor wanted her to stay to convalesce until she was a little better/ steadier on her feet whatever.
Goodbyes were difficult and particularly upsetting for all concerned, so we generally drifted away without a goodbye or any fuss. This all sounds very cruel and sad, but we often saw MIL in the lounge with the other residents chatting away or laughing before she saw us. The sight of us visiting was what turned on the tears and fretting, so I was fairly confident that she was content there for at least some of the time.
I hope it all goes well for you - it's all very hard.