Yes, Mrs Bertie is an irremovable fact of Trennels life, like the Maynard's Anna, who refuses to marry because she wants to stay with Joey. (Though, mysteriously, Anna only gets introduced in Exile, doesn't she, having somehow got herself 'no one knew how, smuggled out of Austria to reach her beloved Fraulein Jo' -- did we meet Anna during her CS days? Did she have a particular adoration for Joey? or is this EBD retrofitting a way of giving Joey a 'faithful handmaiden' equivalent to Marie, who followed Madge to the Sonnalpe, I think?)
And of course who presages Jockel, who somehow managed to get himself and Rufus out of Tyrol and across Europe and find Joey (to whom he is also devoted, obviously).
It always reads a bit weirdly to me that two school servants, one of whom has what sounds like quite pronounced learning difficulties, are both somehow separately inspired by their love of a former CS pupil to escape from Austria ('no one knew hoe') and find her, one with her (enormous, striking, difficult-to-feed and smuggle) Saint Bernard into the bargain.
Especially as immediately after the account of how Jockel managed to escape with Rufus, we get an account of Friedel and Bruno's imprisonment, torture, and their difficulty in escaping, even though these are two intelligent, well-connected men with family influence and money behind them.
You're left with the impression that it's easier to escape the Third Reich if you're solely inspired by the love of Joey!
(Maybe the Marlows put up with Mrs Bertie partly because of Doris and her fabulous dressmaking?)