Subject-Verb agreement with compound subjects

[Miles]

Carol:
> You're correct that the verb in an "or" phrase agrees with the nearest
> noun or pronoun. However, I also agree that "Either he or I am going"
> sounds ridiculous, and "Either he or I is going" is not only equally
> ridiculous but incorrect.

Miles:
May I add something that I feel to be similar? It's from the PS movie, in 
the first flying lesson Madam Hooch says:
"I see a single broom in the air, the ONE riding it will find THEMSELVES out 
of Hogwarts before THEY can say 'Quidditch'!"

Doesn't it feel odd for a native speaker to switch to plural, although the 
sentence starts addressing single persons? I asked a native speaker who 
assured me that the sentence is correct, because both male and female 
persons are addressed.

Now, this is a tricky situation in all languages that know the distinction 
male/female (so most probably all languages in the world). The French would 
use the male form as long as at least one male person is involved, in German 
the "neutral" grammatical phrase would be the male one. Recently, German 
speakers would try to include both sexes, for example with an 'oder' (or), 
which quite often is not really elegant.

Would this be a sentence that ought to be rephrased to be neutral concerning 
sex (gender)?

Miles, becoming interested in grammatical structures quite late in his life 
;)