Offline wallet (Electrum) + Trezor not signing transactions
Well, yes, sortof.  Signing a transaction requires the scriptPubKey from the transactions that funded the source addresses, and if you don't have a copy of the blockchain there's no way to get that information.
I believe Armory handles this by including the necessary information in the data it gives you to take offline, but Trezor apparently does not, and does not considers this use case &quot;redundant.&quot;

that's normal. Trezor requires the list of input transactions.  your offline wallet did not have them and tried to fetch them.
we will not fix that. the list of previous tx will no longer be needed with segwit.
Electrum issue

If you wanted to work around this issue, you could modify the Electrum plugin to load the necessary info from, e.g., a text file instead of trying to use the internet.  The place you would need to change is shown in this post (.../plugins/trezor/plugin.py).