I don't have input but to say .. as if you tell someone in life..
If you have done all you can and they are doing all they can, then you know you there's not much left to do. The next question to ask what can you (or they) deal with and will what's left. In this case there's NO easy answer, only you know what Molly sees and does and if making that choice means putting her in a better place you have to lay your thoughts aside and give her what's best. No one (including her) will think any less of you.
I know im going to be facing that with my 15yr old ball of fur at some point one day and at this point now I hope to look back at this post and think of what I wrote and know when the time comes I will have made what's best. Good luck to you.