It's for this reason that it is necessary to save completely the directory of the byteballs, not just seed
Yeah, but you must save it after each transaction lol. It's very hard and here can be some other problems. Like an empty balance
You must not, if you are using multisig:
... private assets, such as blackbytes, are stored only in the wallet (exactly because they are private) and get updated every time you send or receive a private asset.
To secure against loss of the wallet, I recommend that you use multisig. For example, 1-of-2 multisig with one wallet on the phone, the other on a desktop computer. This way, the private assets will be automatically copied to the two devices.