So, on the icons in the aircraft selection menu: that’s answered here. Basically just look for the generate icons button
And on the HD mesh: I would definitely try the default mesh first. As you know, the HD mesh is older, and there are some places (I think mostly in the US) that have very nice buildings and stuff like that in the default scenery, which could disappear when you switch to the HD mesh.
In the Netherlands, the HD mesh definitely looks better than default.