Is yr T on holidays or is she doing further twining / attending workshops / furthering her knowledge?
When she said to you she wants to help you work on finding support when she's gone, I read it a little differently than you do - I'm not sure she was meaning 'I can be replaced' but instead was acknowledging the very real need you have to feel supported when she is not around; AND she wants to HELP you to work out how you might begin to find other support, for when she is away.
That's my take on it anyway.