XPress doesn't add anything to the title. The title gets piped into the native XenForo <xf:title> tag, as is necessary to display it in XenForo templates, and hence the Board Title gets appended by XenForo.
I'm not sure in how far it doesn't respect Yoast titles. It's working as it should for me. XenForo - as on every other subpage - appends the board title to the URL. If you don't want it doubled, you'll need to remove it in Yoast.
The message is phrased, so you can customize it, however the numbers provided are static. You could edit the template if you want, all numbers needed are accessible.
A matter of preference. It's just as correct as the other one.
There's no original widget, so not sure what you are referring to.
Independently of the given problem, updating an additional table when already performing a write operation is a neglectible cost. There'll be no feelable impact.
Running a query of that size will render your site slow for a few moments as your number of posts, threads and users grows.
We can make a big discussion out of this if you want, but the tl;dr is that we don't blindly copy-paste suggested bug fixes but rather check the software and code if that is actually fixing the issue and not just bandaging it on the surface, so no errors follow up, similar issues may also be...
I'm not sure what else you want me to tell you. There's a cover that has been applied to a resource. You can delete it in the ACP cover browser most likely. I can't tell you how it got there, all I can tell you is that it is there, and that it is, what is causing the error.