"The Soros' " would be correct, I believe it is this correct between both British and Americans. You have to put an apostrophe at the end when referring to a last name as plural if that name ends with an 's'. Otherwise, if the last name were something like "Barrymore" you would state the family as the "Barrymores".

Your structure and grammar is fine, except you use the words ''American'' and 'British' as if they are languages. I am assuming you mean American English and British English. Both U.S.A. and Britain hold English as their standard language, but sometimes they are separated as such because of different terms and slang that are not common in both natives vocabulary. Basically, I suggest using "English" when you are referring to the general language and either "American English" or "British English" when referring to the individuals.

Both would be acceptable, but "It was an exciting moment to be remembered" is used more commonly. Though it's very vague advice, I hope I helped.