is verification only done during signing and prior to this the green ledger light on Electrum is an indicator only (trusting that the device holds the corresponding keys because the device says so)?
I think that's an accurate assumption. I'm no programmer, but it's my understanding that Electrum can only assemble the transactions, then they are signed internally by the Ledger (any hardware wallet) then the signed Tx is broadcast by Electrum. Electrum has no access the private keys, not even when the hardware wallet is connected.