Sorry, in German:

Die genannten Instrumente der Beeinflussung führten zu einer tatsächlichen Zustimmungsrate von geschätzten 80 bis 90 Prozent [vgl. für die Kommunalwahlen 1989 Kloth 2004, S. 19] – aus den eingangs genannten Gründen für die SED-Führung unakzeptable Zahlen.

Source: http://www.mdr.de/damals/eure-geschichte/themen/wahlen106_page-3_zc-c0952f36.html

Source for the numbers: Vom »Zettelfalten« zum freien Wählen. Die Demokratisierung der DDR 1989/90 und die »Wahlfrage«, von Hans M. Kloth

This is impossible to answer, as the whole process of voting was under strict supervision by the state.

While there was in theory a multi-party system, all voting was done via a single proposal ("Einheitsliste"). This means that all candidates from all parties would be found under the heading "Nationale Front", and there was no alternative. Also, the results were pre-determined as well. Which party would get how many seats was determined before the election.

This would leave the option of casting invalid votes. However, this was not really an option as well, as the voters were kept under close supervision during the actual casting of the votes. The amount of official invalid votes was between 0,06 and 0,54 percent between 1949 and 1989.

The only election for which at least some independent data is avaiable is the communal election of 1989. Officially, 99.91 percent of the votes were valid, and 98,85 percent were in favour of the National Front. The counting was then watched by the public, and in several voting districts, far more votes against the Einheitsliste were counted than mentioned in the official publications.

In short: The official figures were certainly inflated. Most people nevertheless voted in favour of the Nationale Front, as doing anything else could have led to problems, and would have been futile in any case. There is no way to tell how a truly free vote would have ended.