A British newspaper, News of the World, on Sunday, January 27, 1918, quotes Kuhlmann of the Reichstag as saying: "After the arrival of M. Trotsky, the friendly spirit which was exhibited on the Russian side during the first part of the Negotiations completely vanished. The Russian gentlemen shut themselves hermetically off, and the entire nature of the negotiations became totally different."
The same newspaper on Sunday Feb 10, 1918 has a report from Reuters saying that: "Trotsky and Kuhlmann seemed to approach very near to breaking point. A dispute arose over the origins of certain alleged falsified telegrams of the Petrograd Telegraph Agency. Kuhlmann... expressed doubts as to whether the Russian delegates really desired the success of the negotiations."
News of the same day says that Kuhlmann was negotiating with the Ukrainians without Trotsky.
In the Marxist Internet Archive there is Trotsky's: My Life
CHAPTER XXXI
NEGOTIATIONS AT BREST-LITOVSK
Journal articles I have looked at from the mid 90's say that Lenin pushed for acceptance of the treaty, Trotsky accepted this and his group in the leadership abstained, while Bukharin wanted to continue armed conflict with the germans. If you have access to JSTOR, an online journal database, you can search for articles there.
The articles do state that there was hope that the proletariat in Germany would help the Russians, though i've found no other word on German leftists' part in the conflict.