Wait, I'm not supposed to have ten windows of my Cashu wallet open?

I'm not seeing how a merkle tree helps here is what I'm saying.
Not quite sure what you mean.
Because Nostr is going to take a long time to develop and build out protocol consideration for different things. Not everything with potential is going to grab everyone's constant attention early on. Attention is a precious thing.
Until shit like this happens on Nostr, its gonna be a niche thing: https://twitter.com/KKArts09/status/1625322537180864513?t=r0h6hgF7uAFkC5_EUdt8SA&s=19
That would need a very thought through scheme for forward secrecy for messages and an airtight scheme for never using a Nostr identity key twice. Otherwise youre going to likely wind up creating a lot of useful metadata for regimes.