Slitherine

It's an all-Naval Scenario and this Marine Leader shows up to be attached to a ground unit. Of which there aren't any. So his portrait just sits there in the top right. Bug?

And he's still there for the Coral Sea, again, all-Naval

Any commander icons in the top right are unallocated commanders.

If you have any core ground units in your OOB you can attach him to them.

Otherwise he will hang about waiting patiently.

One thing I would like to ask is is it possible to re-allocate a commander to a different unit later, & if so, how?

I guess if you have unattached land commanders in a naval scenario you can't "get rid of them". Probably need a button to hide/minimise the portrait pictures.

As for re-allocating, select their unit, open the info panel ("i" button on its picture in the bottom UI), then you'll see the commander's portrait there with a red X at the top right corner. Click it to detach. It takes 1 turn before you can reassign to another unit though.

Thanks for the useful info on commander re-allocation.