You emphasize the before, importing an address cannot be done after funding the paper wallet?
Of course you can always import any watch-only address. The only benefit from doing it
before funding it, is that you don't need a rescan.
If the address is funded already, any import requires a rescan to show an accurate balance. Importing unfunded watch-only addresses saves time, that's all.
And if I hold private keys with some UTXOs at the corresponding address in the bitcoin-qt wallet and one or several watch-only addresses as well and this is all intermingled together to a single balance, how can I differentiate what values are spendable from the bitcoin-qt wallet and what belongs to the paper wallets?
Bitcoin Core adds a new column "Watch-only", like this:
