My third paragraph clarifies.
As for the second point, why should the entire team have the same nationality. Just because they all work or are associated with with same institute does not mean they share the same nationality.
I didn't explain that too well. Yes, there could
be mixed nationalities, just about, but it would be most unlikely
. Todays modern science field is bonded together by electronic communication and American, Swiss, Finnish and any other nationality of scientist you can think of all come together to undertake projects. People from one side of the globe travel to work at institutes on the other, it's commonplace.
But, just bin that concept altogether, right now, okay? Trust me.
This is the 1920s. It was different.
There is only postal letter communication for scientists to share vital ideas. Telephone could be used to talk casually but for the passing over of documents we're talking paper and envelopes. And most of that would be by sea. Not even airmail would be around in significant amounts yet. So communication between nations would take days at least and possibly weeks if ocean barriers were in place.
Also in the 1920s groups were as partisan then as they are now and if you think about the secrecy that surrounded Carter's Egypt expedition and how it was an all-British enterprise you get an idea of the way scientists thought then, more so than now I think. Patriotism filtered down to scientific levels much more strongly then.
I think we could consider the team being sponsored by a university in Europe somewhere and perhaps stipulate that at least 2/3rds of the characters in the team are that nationality. That would make historical sense. And there were direct national conflicts of interest going on. A team sponsored by say, a French university would quite possibly not accept German scientists, or Russian. Just a possibility.
Just my thoughts, BTW, in my usual style of seeking verisimilitude, no need to get too excited about them.