Any question answered...

The Wikipedia page should answer most of your questions:

she was president of the People's Chamber of East Germany from April to October 1990. During this time, she was also the interim head of state of East Germany, holding both posts until the state's merger into West Germany in October. She was the youngest, only female and the last head of state of East Germany.

[...]

In the general election of March 1990, the only free and fair election ever held in East Germany, she was elected to the People's Chamber, which on 5 April elected her its president. On the same day, parliament also abolished the State Council, the country's collective presidency. Under the Constitution, the president of the People's Chamber was ex officio vice president of the GDR; as such, Bergmann-Pohl assumed the role of interim head of state as well. In that role, she presided over the People's Chamber formally petitioning to join the Federal Republic of Germany on 23 August, as well as the overwhelming approval of the unification treaty on 12 September.

https://en.wikipedia.org/wiki/Sabine_Bergmann-Pohl

I guess she would have been something like de facto, if not de iure head of state, but don't take my word for it, as I don't know much about legal things.

The Volkskammer was not a democratic institution, as elections to it weren't free, and so you can't really compare the role(s) she held to the ones you cite. I don't know in detail what official differences existed, as opposed to the real differences. She was just one of the shits that sold out the GDR to western corruption.