I don't like the idea that words of either British or American origin corrupt the language when they cross over- it's only natural that words will move around. When I was a child, movie was regarded as AmE, then it started being used by some speakers and it grated with many, but now it just sounds normal to me, though it exists alongside film. There are some that are crying out for standardisation IMO- like mobile/cell phone, where it would make sense to have a single term as it belongs more to International English than national variant IMO- maybe queue comes in that category too.
Losing shades of meaning can be a problem- the distinction between may have done and might have done is probably critically endangered now, which is sad. As we don't distinguish holiday/vacation, I will feel less of a loss than you.
If someone uses momentarily in the AmE sense online, they are sure to get comments from people accusing them of selling out to AmE, an attitude I don't like much- words move around, some stick, others don't. Bill Bryson's piece about older American terms in English surprised me as I had no idea many of them came from AmE. Once the shock of the new wears off, the arguments fade away. In the 1960s, some conservative BrE speakers maintained that the Beatles should have sung a true BrE song She loves you, yes, yes, yes- I doubt many would still be arguing that one.
With the new media, access to other variants is only going to increase and this traffic will increase. I had no idea that go missing was BrE until I read the article, but that sort of term is easy to move around.
Gobsmacked was regional slang that moved south several years ago and was much overused- it's way past its sell-by date here too. It is a good word and is sure to make a comeback in a few years. BTW I have ginger pronounced to rhyme with singer when used pejoratively, possibly influenced by the word minger for an ugly person, though I haven't heard that used much of late.