“were” ought to be replaced by “was”. A colleague (and superior, unfortunately) maintains that it is correct as written. Who’s right?
NB: exactly what x, y and z are doesn’t matter. They’re all singular objects.
My argument is that “all” is functioning as a singular, in the same way that “what” does in the sentence “What we need is more badgers”, which the dictionary definitely prefers over “are” in this case. But I’m quite prepared to bow down to superior knowledge. (Just not my boss’s. )
IANAEP, but I believe that “were” is correct. The sentence implies that "All the objects [we took with us] were an x, a y, and a z. " If your boss is still not convinced, point out to him/her that it can be rendered alternatively as “The only objects we took with us were an x, a y and a z.”
And in your second example, you need one thing. What is that thing? More badgers. So your sentence is basically the same as “The thing that we need is more badgers”, which is why it’s “is” and not “are”. The “what” has nothing to do with the “is”.
“were” is correct. Syntax requires the verb to agree in number with the three items listed.
[If it matters, I have honours degree qualifications in English Language and English Literature, and my courses included grammatical analysis to phoneme level.]