Testes

if I don’t change the custom message and write a custom excerpt had been written then Publicize would send that to Facebook in the post rather than the one it generates with the [??? more words] suffix.

That’s something we’d like to implement as well. You can follow our progress on this here:
https://github.com/Automattic/jetpack/issues/1183

2) Twitter
[…]
I’d be interested to know if this is expected behavior.
[…]
Basically I don’t want it to do it, so apart from not having images in the first 256 characters is there any way of avoiding this?

That’s expected behaviour for now, but we’re working on improving the image selection process, and we’re also thinking about adding an option that would allow you to disable the image upload process and only rely on Twitter Cards instead. You can follow our progress on this here:
https://github.com/Automattic/jetpack/issues/565