(apologies if this appears twice, first submit disappeared without error or message)
ok, been struggling with this for a while (ok ages) and hunted for info or tips but couldn't find anything.
so time to ask for help.

For a while I've been making custom maps for my gpsmap62 originally as kmz, then jnx files, then merged jnx, and most recently as ".img" maps to layer on top of the built in maps. (i.e. I can render high detail footpath maps using mobile atlas for the areas I'm going to run in and convert with mobat2jnx->jnxcustomizer->jnx2img, ok not the slickest of workflows but does the trick)

As jnx files these work ok when the level is set appropriately and only appear at (e.g) 500ft zoom level. But as img files they render at all zoom levels and so take precedence over any other maps.
(ideally I'd like to be able to merge these into the same img map so I can use them to plan out and work inside basecamp with the 3d dem, able to zoom in and out to high detail and out to basic mapping etc without needing to toggle the maps)

So the questions:
1. am I doing it wrong, is there are better way to create an img file or should this sequence work and I've got something wrong somewhere.
2. is there any tool I haven't found yet that can adjust the zoom levels and visibility levels of maps inside .img containers?
2b. I looked at the current .img file reverse engineered documentation to try to see if there was a simple header setting for zoom level visibility, but couldn't see it other than the "precedence" value which doesn't seem to be correct. Is this data buried further down inside the GMP files?

Many thanks