That might ameliorate some of the GUI display issues but it wouldn't solve the problem of watch-only-address txs being included in the staking calculations - which happens in the app's core routines. The (optional) GUI wallet code in src/qt/* is just a visually-rich presentation of the data obtainable from the core RPC API provided by src/bitcoinrpc.* - which would need to be refactored (read: “completely rewritten”) to handle >1 wallets.

Cheers

Graham