The trouble there though is the question why it is that way when another configuration is conceivable.

It's rather funny that you can see the very same problem with what I've said that I've trying to get over to you. Nothing
you've said about a 'necessary entity' answers this question.
Think about it.
Does what you're saying stop space time being necessary because it could be no other way. I would say no .So yes you are arguing for necessity and against it IMHO, because you are saying space time accounts for contingency.

Why, oh why do you continually look at everything I say as an argument for or against something? I specifically said "
I'm not arguing that this is the case, it's just logically possible."
The universe (space-time)
might 'just be'. That's just a logical possibility, given what we know at the moment. I care not one jot about 'necessity' because I can't see how it can possibly make logical sense and you seem totally unable to explain how it could. To be fair, I've never found anybody else who can either.
I'm open to being persuaded, but I cannot see how it makes sense and nobody I've discussed it with yet seems able to explain it logically.
Coming to something rather than nothing, Does nothing exist? or better still does non existence exist? And if it was a choice between nothing and something, something had to actualize that choice, since nothing is incapable of either choosing or actualisation or effecting that decision.
Therefore we have an unactualized actualized, and that which has been actualized.
Is this load of theobabble supposed to mean something?
In terms of no entities being contingent, I'm not sure who believes that and operates on it and would certainly say the burden of proof is on someone who suggests it.

I think it's ambiguous, because if the whole universe is a 'block' that just is, then nothing in it could not exist.
Again, I regard the argument from contingency to be total bollocks.