As to what Agatha Christie would have thought - well, in 'Mrs McGinty's Dead' her alter ego, Ariadne Oliver is having a book adapted into a play by Robin Upward, and I think we can assume that her complaints are based on Agatha's own experiences....
"But you’ve no idea of the agony of having your characters taken and made to say things that they never would have said, and do things that they never would have done. And if you protest, all they say is that it’s “good theatre”.
"‘The whole thing’s a nightmare! How would you like to see a big black moustache stuck on to Superintendent Battle and be told it was you.’
Poirot blinked a little.
‘But it is a nightmare, that suggestion!’
‘Now you know what I suffer.’"
And the adaptor's take on it:
"‘But darling Ariadne, the whole point of the play is Sven Hjerson. You’ve got an enormous public who simply adore Sven Hjerson, and who’ll flock to see Sven Hjerson. He’s box office, darling!’
‘But people who read my books know what he’s like! You can’t invent an entirely new young man in the Norwegian Resistance Movement and just call him Sven Hjerson.’"
Sarah Phelps, like Robin Upward, obviously thinks you can...