Thanks to everyone for the wisdom! Since I just joined this forum, I'm not able to thank or groan yet using the icons... Nothing to groan here though, just thanks all around!
@JoMIiFa, thanks for confirming the half fare card thing, was not 100% sure about that. As you point out, the monthly usage is likely to be lower than my first estimate but I'd prefer to budget on the side of caution; I can imagine a few more trips might be useful compared to a single person. Also, thanks for pointing out that I am double-counting on the train commute. I would never dream of commuting by car given the 20 min direct train ride.
On the cost of owning a car, assuming I buy a 2nd hand 2006 Opel Astra 1.8L (100k km on meter) for 10kCHF and pay it over 5 years, this equals 2,000 per year. Add insurance for said car for 700/y, add fuel 1,980 (100L per month at 1.80per L), then teh hardest to estimate is servicing so lets say a contingency of 2,000/y. All this adds up to 6,680CHF/y.
@MathNut, I can't add add the parking to the total above as I seem to be forced to pay for a parking space in the flats I'm currently looking at irrespective of if I own a car or not!!!
@VillageIdiot and slammer, forget to say that I had added ski-rack and GPS to a staionwagon car rental. But you are right, it could be lower.
@tildaoz, agree that when you don't own something you do not need to keep the utilisation rate high to justify its purchase. We may very well end up using the car much less than in my estimate. But again I'd rather err on the side of caution.
@adrianlondon, agree, hence the short trips budgeted for Mobility car and the extra car rentals for the longer trips.
@kri, thanks for the very relevant experience shared. Uncanny that you should have ended up with a total of about 6,000CHF /y! However, I think that you could probably reduce this significantly in a best case scenario following some of the above principles.
@HIOA, were dead set on not living in Geneva, so I'm stuck with the higher train commute cost...
In conclusion, the new total for public transport + car-sharing + car rental racks up at: 2,895CHF/y. Which is nice compared to the owning option coming out at 6,680CHF/y. So, based on this thread, we will defer the decision of buying a car to at least 6 months down the line from when we move.
Incidentally a nice little statistic I picked up: average annual cost of owning, using and maintaining a car (across all personal light duty vehicle segments) in France is 6,000 EUR (15,000km/y). That's all the possible costs you can think of being included.
Thanks to everyone who contributed!