In general, you are always allowed to take the currently fastest connection to your destination. This follows from section B 5.3.1 of the VBB fare regulation:
Diese Einzelfahrausweise (außer Kurzstrecke) berechtigen zu einer Fahrt mit beliebigem Umsteigen in Richtung auf das Fahrtziel über den reiseüblichen oder durch die Fahrplanlage bedingten Weg.
(Those [e.g. Berlin fare area] single tickets (except for short journey tickets) entitle the holder to a journey with any change in direction to the destination via the usual route or the route determined by the current timetable situation.)
Only if you wish to interrupt your journey, the time limits become applicable:
Fahrtunterbrechungen sind innerhalb der nachfolgend aufgeführten Fahrzeitbegrenzung beliebig oft gestattet [...] im Tarifbereich Berlin 120 Minuten.
(Travel interruptions are permitted as often as desired within the travel time limit listed below [...] in the Berlin fare area 120 minutes.)
So, in short: if you get delayed on your journey for reasons other than your own fault, you can always travel to your destination.
However, what you plan on doing is not possible for another reason: The stop "Kähnsdorf Dorf, Seddiner See" is located in zone "6049 Kähnsdorf, Dorf", which is not part of the "Berlin C area" as evidenced by the VBB zone list. Note that there is an unrelated concept of a "Potsdam C area" which that stop is in but that's something else. So a VBB Berlin ABC ticket is not valid for a journey starting in Kähnsdorf. For the journey from your question you would need a "VBB R2 (Regionaltarif bis 25 km)" ticket for 4.30 € which allows you to travel to your destination with the fastest currently possible connection.