If there's something Mercy needs

I agree and I think her issues lie mostly with Valkyrie (and are actually easy to fix):

Most of those are general tips for game-sense, coordination and communication, not really a Mercy skill but a Overwatch skill, applies to any if not all supports and tanks, thus it’s not really enough to alter the skill gap.

The only one you mentioned that is actually a Mercy skill involved Valkyrie, getting a pick on Ana/Widow, and Valkyrie gives you insane self-heal, free flight, infinite clip and 100% projectile speed, so the skill demanded there is also on the lower side. Too much so I would argue.

It would be nice if Valk gave increased damage rather than infinite ammo that enables spam.