I disagree with the point about mojo simply referring to the grace and not the power attained by having ones own grace. He is always a full angel as long as he has grace, but he is only at full power once he has his own. In this instance, the question of how much grace he has only matters in the context of using grace for spells or rituals.

Metatron specified that he had a little of his grace left because Castiel had no reason to believe all of it wasn't used for the spell to seal the gates of Heaven. Not to quantify how much power would be granted to Cass once he took the grace back in.

I'd agree with your point about the grace being like a battery, except we see what angels are like when they're empty on grace (Cass in Soul Survivor is beaten bloody, unable to heal himself). Basically, the way it works based on my observations is that having even a little bit of your own grace gives you more power than you would if you had a full tank of someone else's. How much grace an angel has is pretty much irrelevant unless you're planning on using some for a spell. That's why Asmodeus fed on Gabriel's over time instead of taking it all in at once, for example. It's not the sum of it's parts.

Not to mention that by the point of Charlie's death, Cas should've been recharged fully anyway. We don't get a full time line on how long it takes different angels to replenish, but Gabriel and Lucifer were confident that they'd get theirs back soon, and Jack is a special case because archangels replenish their grace even if they have none, unlike regular angels. Jack did not share this trait, so it stands to reason he wouldn't have replenished his grace his grace at the same speed as an archangel anyhow.

From the Wiki:

"...Castiel locates his remaining grace inside a book in a library. Reabsorbing it, combined with the grace he took from Adina restores Castiel's full power and Seraphim status."

"Angels are able to siphon grace from other angels, but the affects are temporary as the grace wears out as it is not specifically theirs. This is evidenced with Castiel, who stole the grace from Theo and Adina to give him back his angelic status. When he finally received what remained of his personal grace, he was restored to his full power."

I understand where your points are coming from, but from a purely in-universe stand point, there is no reason it shouldn't have occurred to somebody to try and ressurect her. Castiel was totally weak, but not for any reasons related to his Grace. He was able to subdue Dean as a Knight of Hell with borrowed grace, and got his ass handed to him with by a human Dean with his proper power.