The gem at Old Ashton is the very last piece you can get for Aiyana's weapon. She mentions in a dialog, but you don't know where exactly it is yet at that point. You have to gather the others first, then Aiyana will give you the info, where you have to look in Old Ashton.

I'm not aware of such a bug for the vanguard battles. Do I understand it correctly, that you have the icon at first, and you can play the first turn normally, but after the enemy finishes his turn, your selection icon becomes invisible?
Are you playing on windows? Do you play the a recent game version? Have you tried if it happens in other vanguard battle maps as well, or if it's only in one?

The bonus scene with Lace and Irinlia happens when you have both of them on their bad path. When Lace is on her good path, you can't get that scene.