...or a extra processor from a Series 40 phone, only to handle the calls, and 2 simple Buttons at the bottom of the Device.

Simple and cheap solution! Yes, they have to develop how to communicate to this processor... But I would say it should not harder than the handling for the proximity sensor or the camera. And they don't need the software tuning and the not working bad tricks anymore. A solid solution.

They could still use the standard linux part to show a ui, incomming call messages, and so on... but everything handled over d-bus and the driver between. But nothing critical on the software side. Everything else, the "ringtone" and "the Buttons" are hardware side handled inside the cheap Series40 Chip so that we are able to answer a call even if the kernel are running amok!

But dreams could take sometimes a little bit longer...