See (Start Here) Frequently Asked Questions Eeschema section.
Edge.Cuts may be OK, but it’s common to use footprints for mounting holes. There are a bunch of them in the KiCad library. It’s a matter of religion to decide whether to use a schematic symbols for them or keep them only in the layout.
It shouldn’t be, and if it is, it’s a problem only for the manufacturer (unless you try to panelize yourself and silk goes to other board). They should know what to do with it. On the other hand you can copy the part to your own library, possible to a project specific library, and modify it. Some people never use footprints directly from the standard library.
I would recommend adding at least a diode to the barrel jack to prevent wrong polarity. Better safe than sorry (although I don’t know what safety measures the module includes - you may want to add for example some esd protection).