Yes, Mrs Croft was really an invalid, and was not pretending. Chapter 19:
> ‘Hello-ello-ello,’ [Japp] said. ‘What’s this? An old friend! Milly Merton, I declare! And at your old tricks again, my dear.’
>
> He turned round in an explanatory way to the company, disregarding Mrs Croft’s shrill protests.
>
> ‘Cleverest forger we’ve ever had, Milly Merton. We knew there had been an accident to the car they made their last getaway in. But there! Even an injury to the spine wouldn’t keep Milly from her tricks. She’s an artist, she is!’
(Blink and you’ll miss it!)