Can Evelynn shapeshift?

She has at least some restricted form of shapeshifting. Her unstealthed apearance is not her true form. When she cloaks she takes on a different form which might be her true form.

At least this makes her most of her skin much more like to be canon.

Yes, but it takes conscious effort on her part.

The demonic form her rework takes in game is her "true" form, which she takes because there's no point in taking an attractive form to lure your prey in when your prey can't see you anyway.