Can a user sync the other wallets as well?

The user will be able to set a wallet for BTC - ETH - LTC when withdrawing. The wallet address will only be used to transfer to the user the amount desired.

could you give some hints on the process when a user received funds in a virtual wallet but then decided to sync his?

When Steem is synced, all the transfers go instantly to the wallet. For all the other coins, the transfers are processed within 72 hours.

is there going to be another tiny note that would display the potential value of the votes?

The upvotes, as they are turned into liquid STEEM by the system, will be shown under STEEM. We can definitely consider adding an additional note to make that clearer.