As I understand it, none of the money is refundable if you're rejected,
Correct. You pay for them to process the application, not for a particular outcome.
and just because your EOI is chosen doesn't mean you will get Residency (as you have to then prove everything).
Also correct.
Is it unlikely that I would get my EOI selected if the Immigration Official didn't think that my argument was a valid one?
Well, it doesn't usually work like that. The EOI is selected by computer, on the basis of the number of points you claim. Then clerks manually look over the EOI, to see if it
looks as though, from what you've said, you had a right to claim those points. Understand, from everything I've seen from other people on the forum, they're not making any judgement on the validity of your claims at that point - they're just checking that, for instance, you haven't claimed for your qualification without its either being exempt from assessment, or having an IQA for it. If it all LOOKS okay, you'll be sent an ITA, which you have to return with all your evidence. It does SOUND as if you were being told that those people, who don't have authority to judge whether you meet the requirement, just might consult a higher authority, but it isn't what has normally happened. In most cases, no judgement has been made by an official with authority to judge whether or not someone met the requirements at the EOI stage - that didn't happen unless and until they submitted the ITA, with attendant costs.
I would have over the 140 points requirement
If you have what is counted as a skilled job in NZ, you only need a minimum of 100 points for your EOI to be selected at the next draw.
I can't advise you - it's against NZ law. If you want some direction, you need to talk to an LIA. There are two who post on the forum, Karen Phillips and ChrisMwn. They wouldn't do anything they would have to charge you for without notifying you and getting your agreement first. If you send a PM, they would no doubt tell you if they think you have a reasonable hope of not wasting your money.
Did you have a look at the idea of submitting your case as a translator?