2ndly, will they treat foreigners(I am an Asian Chinese) differently ?

Once, I went to the Paris Opera House box office but my every enquiry was given a nil or zero by the ticketing officer. I loved Paris so much until this experience during my 4th night which was my last night, luckily not earlier.

I agree that a 5-hour opera is going to be a challenge if you are standing for the whole performance. However, I believe that some opera lovers might indeed be prepared to stand for so long in order to experience a whole opera such as Die Walküre at such a magnificent opera house. And, of course, some people might not be able to afford seats. If you decide to see another opera in the future, then perhaps you will not have such a full schedule next time (seeing a famous opera, having dinner, visiting the city, etc.), which will mean you have both the time and energy to be able to see the whole opera. A shorter opera and/or a more manageable itinerary would also mean, hopefully, that you don’t come and go during the performance – something which I believe should be avoided out of respect for the singers and musicians, who do their best to deliver a whole and continuous performance to an attentive audience. Whatever you decide, I wish you more successful opera visits in the future.