How to determine the number of the noun phrase 'a world of + plural noun'?

I haven't seen specific guidance on this, but I wanted to make an answer post to address something I think was missed in the list of examples in your question.

And with the collapse of those ''walls'', a world of opportunities is opening before us.

Here, it's not necessarily the case that "a world of opportunities" is just being used as a synonymous expression for "a lot of opportunities". The noun world in English can be used metaphorically to talk about a environment or ecosystem: access to a new "world of opportunities" isn't necessarily just access to greater quantities of the same kinds of opportunities that were already available.

The interpretation of "world" as a noun with actual semantic meaning beyond "a great quantity" in this speech is supported by the context. Anyaoku's speech continues in the following way:

Opportunities not just to put a definitive end to the "armed peace" and its incalculable wastage of resources, both human and material, but also opportunities to step up the fight against humanity's enemies, old and new; political instability and local conflicts; hunger and disease; drugs and the deteriorating environment. It promises to be an exciting world but it will not be all sweetness and light.

Note that the word "world" occurs again in a context where it is not followed by "of", and where it unambiguously acts as the head of a singular noun phrase.

I think a good first step when analyzing an expression of the form "a world of [plural noun]" is to check whether it makes sense as a noun phrase headed by the noun "world" (which has metaphorical senses that don't just express quantity). This is basically the same as checking to see whether a phrase like "an ocean of flowers" is being used to refer to a literal ocean, in which case it would obviously take singular agreement: it's a bit more difficult for world because its meaning as a noun is more abstract, but I think the same principle applies to both.