mempool.space's accelerator needs an account and it need to be activated by them,
mine which was registered about a month ago is still in their "waiting list".
So either they don't have the connections yet, or they don't want to earn money? That's weird for such a service.
You can just create a CPFP transaction to the unconfirmed transaction that's sent to wallet B.
I discussed this with OP, but he doesn't feel confident handling private keys and manually doing this in another wallet. Wallet B can't do anything.