Hi dpakdee, and welcome to MySwissAlps!
>> We're considering 2nd October: Geneva-Zermatt-Geneva
That's a long day: 3.75-4 hours travel each way, plus whatever time you spend in Zermatt. If you could stay in Montreux instead, that would cut at least 2 hours off your return travel for the day, and would also reduce your travelling time to Grindelwald the following day.
You can check travel times by using the Swiss Timetable:
>> 3rd Oct. Geneva-Montreux to Interlaken/Grindelwald via the Golden Pass Line (I assume the Line is the same whether going from Lucerne to Montreux or the reverse?)
Yes, the line is the same in both directions. If you are starting from Geneva, put "via Montreux" and "via Zweisimmen" into your timetable search. If starting from Montreux, just put "via Zweisimmen" into the search. Otherwise the options presented will favour the faster routes, rather than the slower, more scenic Golden Pass route.
>> 5th Oct. Interlaken to Brienz for the Brienzer Rothorn sream cog-wheel train ride and/or the cogwheel train to Schynige Platte, OR Jungfraujoch;
Any of those trips would be possible if the mountain tops are not in the clouds. You can use the Swiss Timetable to plan your trips and decide what is feasible in one day. The webcams will help you decide if weather conditions are suitable for a visit to the mountain tops.
Webcams for the Jungfraujoch and Schynige Platte
Webcam for the Brienzer Rothorn
>> Is Montreux to Lucerne via Interlaken the same as the Golden Pass Line which is described as Lucerne to Montreux via the Bernese Oberland?
Yes. The full route is Luzern-Interlaken Ost-Zweisimmen-Montreux (or vice versa), with a change of train at each of the intermediate stations. It is sometimes also necessary to change trains in Spiez. You can read more about it here: