Regarding issue #2:
This is an intentional design decision. Logic goes like this:
If MBTiles overlays appear in a Chart Group that contains other types of charts (e.g. ENC
or RNC), then MBTiles are disabled (red-X) intentionally on OCPN startup.
The idea is that many (most?) MBTiles overlays are "not suitable for critical navigation", as they often are georeferenced but un-validated photo
So this is a safety
feature of sorts.
I can see an argument when using MBtiles that are not photo
overlays, as your samples show, that they should be loaded verbatim.
But I am not convinced. The little extra effort required to click the piano key might be a mental signal that you are shifting from a validated navigation
chart to something else...
Sadly, MBTiles do not contain any metadata describing the quality, provenance, or even Unit Of Measure for soundings. Quick, what is the UOM for soundings on your sample tiles? I cannot tell, either.
So my opinion is to play this conservatively...