This should not be too much in a healthy situation, and this also doesn't imply that the developers can control the PO. But when the developers need the product owner to clarify or renegotiate the work to be done, it will be an impediment if the PO is not available.

if the documentation is adding to the quality and maintenance of the software this is definitely a good argument for documenting every sprint. Just because that's the way it's done and we don't want change is not a good reason. Tooling that requires is also isn't. And the scrum master cannot demand this kind of things. If the company's definition of done says so, you must :)

If you are not sure, please review the Scrum theory section in the scrum guide

It should be a working increment, but it's not mandatory to release it.

Merging means combining two different versions of the code into one version.