it says you get the charm for a single use. nothing says that use has to be the same round as the gnosis spending. also i would say that any loss of essence for a garou get translated to loss of gnosis.
1) In the Numen write up, it mentions using gnosis to boost a relevant stat which implies at the moment prior to rolling. Using a charm is mentioned as an alternative to the aforementioned ability. It would make sense that borrowing the charm occurs simultaneously. I also wouldn't think it appropriate to use a Heal charm, reform, materialize during rage actions. It would get silly.
2) By making it so essence isn't simply magically replaced by Gnosis to make a charm work, you prevent Garou from getting access to say... Break Reality such that a cliath can't turn the face of an elder Garou into Silver with a static challenge. Or say, a cliath metis unleashing 5 agg blasts, two~shotting and elder bsd without a chop.
Basically, charms that require essence are quite powerful, and while there are plenty of charms useful that do not, balance would be best served by clarifying this distinction. Let's remember, Garou borrowing charms from spirits are from Numen and an ELDER theurge gift.