If I lose my BitBox in the future and the seeds from the BitBox as well, are my coins always gone on the Electrum Wallet?
Or can I just restore my Electrum wallet with the seeds and just connect a new BitBox to it?
If you lost your BitBox device, and you lost your seed backup that is written on paper or metal (that should always be kept separated), than you can't restore your coins anymore.
Be careful experimenting with keys and stuff like multisig setup.
Unless you leak it yourself, send it to some suspicious website, or keep it in digital form on your computer... you should never do this.