Assuming it's the same as in E70, the metering unit is rather easy to access - well, you'll have to crawl under the vehicle, but you seem to be accustomed to it already
, - I'd have it removed and tested first. Sometimes it just gets clogged and a simple cleaning suffices.
Re DEF having a shelf live and going bad - it's a theory that resurfaces once in a while, but I always doubted it. BMW themselves removed the tanks flushing from the required maintenance, and what about all those commercial stations with big tanks, who knows what's the turnaround cycle in those. So, yeah, it can go bad, but it's a matter of a very looooooong time/abnormally extreme conditions.