I figured out how to receive via the Electrum online watch-only wallet, by taking a photograph of the offline QR master public key (MPK) to create the watch-only wallet, which then has the same receive address as the offline wallet.
Still figuring out how to send from the offline wallet with 1 USB, 1 offline computer (the same online computer, but booted with TailsOS live USB and offline with a command like sudo ip link eth0 down or whatever (too much effort to physically disconnect the RJ45 Ethernet plug)), 1 online computer and 1 online iPhone (no Electrum nor Sparrow app).

Also, is there a way TailOS can automatically not connect to the Internet or have the offline Electrum app in a VM/Container/Qube to keep it offline?