Is it possible to use Bitcoin Core Qt as interface for DIFFERENT full node? bitcoind is running on the background, local RPC server is running, all good. But is it now possible to have GUI? To say to bitcoin-qt that it should use RPC instead of going to disk files (which are locked by the other instance)?
No, there is no support for this. It is not an interface which uses the RPC interface, it has much deeper access within the wallet than RPC provides.