It seems to me from memory that you should be OK from that position I came here alot years ago when in London, as a matter of course but from my last recent visit I think visibility is OK where you are, you are right website indistinct about some seating and when I booked rigoletto recently had a similar problem I checked with theatre the old fashioned way by phone. But that was a bit of a job too.

Usually not unless you have a personal connection drop very lucky or are in a wine bar afterwards and they come in gone are the days of stage door jonnys I'm afraid I think security issues have killed off those possibilities now.

First night at an opera was always white tie or black, by tradition and ladies the same formal wear, after that still formal but some thing less grand if so wanted unfortunately as the previous comment anything goes now, I personally as my partner does follow the old rules by preference and its lovely to dress up maintains a standard, I would say however wear what you are comfortable in after all it is at the end of the day the performance you come to see that matters, dress is a matter now of choice hope this helps a little

There are alternatives but not as good

I sat stalls circle right A90 for Carmen and although not central, it was an excellent, clear view. Got fantastic advice from the box office, so I recommend booking through them for expert help. Enjoy!