Well, in the old DOS days it was possible to have third-party replacements for some parts of the OS (like 4DOS replacing the COMMAND.COM). Well, I know it may be a bit more complex now, but perhaps it would be possible to write a replacement files for those of IE, that would enable to use some other HTML engine instead?

Sure, I'll do it when MS release the source code.

It's enough if we know the interfaces that the third-party applications expect and use - that's all that would have to be implemented.

Can we be sure MS does not use internal (undocumented) functions? If so it wouldn't be the first time.

Well, at least we would have the non-MS applications fixed this way (by replacing the "official" interfaces).
And the undocumented functions can be discovered and imitated, too. It wouldn't be the first time.

I think I'll wait for the source code before embarking on the project. But don't hold your breath.