Is it possible to have multiple accounts in MultiBit HD? Is it possible to have multiple accounts within one wallet? Mycelium allows multiple accounts within one wallet, but I don't see how to do that within MultiBit HD.
Only a single account ("Account 1" in myTrezor.com terminology) is supported in MultiBit HD.

This is due to limitations of how the transaction data is obtained from the Bitcoin Core nodes. It is explained in the help in this article in the section headed "Bloom filtering and single HD account support":

https://multibit.org/en/help/hd0.1/how-spv-works.html

Source: I am one of the devs for MultiBit HD.