As @Tom_A said - unfortunately this is a common problem and varies a lot between units. I don’t have a good feel from reading the forums for whether there’s very many people who actually have good (i.e. ~1mm) alignment (would love to know!).
If using a jig isn’t an easy option for you, or if snapmark isn’t available on your machine, here’s another quick technique that might make it a bit easier for you – basically just map out what the error is for a given height across the bed, and then mentally take that into account when placing work. See this linked thread:
FYI – like most of the other threads about camera calibration, I was asked to contact the support email address with the screenshots of the print of the gift of good measure (see the end of the thread). Take a look and see if those instructions apply to you. It appears that the support queue is really busy though - it’s been 2.5 weeks and no reply.
Also, yes that sounds right about the visible area on the camera (or at least matches my experience). The laser needs a lot more area to move around the boundary of where it cuts. Having a larger crumb tray than the cuttable area also lets you put bigger pieces of material in and still have them lie flat.