Explanation of the English in this University of Cambridge graduation certificate

This is simple reversal of clauses: see this question. If it helps, you could imagine a bracket after 'was', and a close bracket after '2016'.

(I also think that using the Free Dictionary to validate the syntax of the University of Cambridge is equally back-to-front, though in a less literal sense.)

One of the answers seems to suggest that it's a case of locative inversion, though I don't see it. I'd suggest that instead you have a complement to the noun congregation consisting of two asyndetically coordinated phrases headed by verbs in past participle form. I.e.,

'holden', the biggest sore-thumb in the passage, is simply an archaic form of 'held'. Cambridge University is very old and so it is surprising that there are not more archaisms.

'Witness my hand' seems an archaic idiom, not really an imperative to look at my hand.