1. check what your contract with the regie says regarding applicability of the general Geneva tenancy regulations that normally govern all the contracts - if there are no exceptions stipulated in your contract, at least the 3 months rent penalty should NOT be reasonable;

2. when in doubt, use the walk-in service of Asloca - http://www.asloca.ch/asloca-gen%C3%A8ve (not sure they are available in English, check) - 90 fee for 1-year membership would buy you advice and if there are any follow-up steps you can assess the reasonability of taking them (and paying for them) after the initial consultation.

3. maybe also check with your employer, especially if it's the UN, they might help

Admin fee may apply notwithstanding the clause as it is only the fee for the early termination.

Good luck...

PS Worst comes to worst, perhaps you could find a replacement to take over the contract and only pay 435 fee but avoid paying the rest? Then you would present them the candidate (best to do it asap though to give a few weeks' advance notice to the regie) and be free. 

Hi, I would check with ASLOCA, there is also a growing trend of finding that the penalty fee violates the law, even is set in your contract, thus, should not be paid. I am now clarifying the issue for mine (CHF 250), will let you know if any good findings will come up. 

I am surprised the diplomatic clause is not in your rental agreement as it is a known fact that UN and diplomatic staff are sometimes re-assigned at short notice.  If you work for a mission or embassy they can perhaps help you .. ASLOCA could also help.