I wonder: (With a little thinking the obvious answers come, but please comment if it is wrong).

If the Trezor in its current incarnation does not send the XPUB key(s) to the MyTrezor server, how can it know how many keys are necessary to send? For example, in the change fork in each account, a new address is used (mostly) each time a payment is made. Let's say I have made 150 payments, and the first 149 change addresses are 0. How would the server know it should ask for the 150th?

EDIT: It could check for each address that it has been used and abandoned spent, so the UTXO is not enough, it needs the whole blockchain.

EDIT2: And after the spent addresses, it can ask for a number of unused addresses, just in case the user has skipped some.

EDIT3: And if the server had the XPUB, it would have to compute the addresses on the server side.