In a sha’as hadchak situation can someone make havdalah on motzaei Shabbos before davening maariv?
Yes, this is fine.
The enactment of havdalah includes two parts: One havdalah in davening and the other on a cup of wine.
There is no specific order for the two parts, the Gemara mentioning that somebody who made havdalah in davening must also make havdalah on the wine (because this was the original enactment), and somebody who made havdalah on wine must also add the havdalah insert in davening (because this is part of the motzaei shabbos prayer).
There is room to discuss whether the ruling of the Vilna Gaon, which is cited in Maaseh Rav, according to which a person should not make Kiddush before davening Maariv on Friday night, applies even to Motza’ei Shabbos.
However, this ruling of the Gaon is not generally cited by poskim, and many do make Kiddush before Maariv, where there is a need (several rishonim state that this is possible, explaining that one of the ways in which Shabbos is brought in is by making Kiddush).
Therefore, it is fine, certainly where there is a need, to make havdalah before davening Maariv.