I decided not to pick up on the matter of the Chinese-to-English translation, because you hadn't supplied the original Chinese text (beyond alluding to an apparent zhǐyào/zhi3yao4 只要, "only/merely/just need"; so long as, provided), and few people would've been able to make much use of it anyhow. So I dealt with just the English examples that were given (and let's not forget that this is primarily an ESL site). It is however hard to make sense of the (English) translation you've given (and whose translation is it, may I ask), based on what you're saying - I mean, for a start, I've not seen many if any examples of the zhǐyào (...jiù 就...) construction in the negative (re. your "I told a Taiwanese student today that this English conjunction, perhaps unlike its Chinese counterpart zhi yao, is better placed in the positive"). I'd understand it more if the Chinese version used or had used something like 在...同时 (zài...tóngshí, glossed 'while' in the Collins/Reverso E-C entry for 'so or
as long as', and with the following example: You can't turn the heat off as long as the system is on. 在系统开着的同时，不能中断供暖。 Zài xìtǒng kāizhe de tóngshí, bù néng zhōngduàn gōngnuǎn. http://dictionary.reverso.net/english-chinese/long
). Anyway, I suspect this could be a case of a mistranslation/picking the wrong meaning, and/or that the English might be slightly more complex or versatile (e.g. allow that 2-to-1 mapping) than the Chinese...but like I say, it is hard to tell given the limited information.
But getting back to your asking that one read the three English sentences as meaning only "provided", there isn't actually anything wrong with them, provided
of course that one comes up with a plausible interpretation (which, we being humans and ever seeking meaning in everything, isn't too hard to do):
"As long as/Provided the plant isn't safe enough, it won't be (put) in operation. So we must ensure that we do as bad an engineeering job as possible. We're nuclear engineers who have seen the error of our ways, the folly of nuclear power, so we will do everything we can to passively resist and indeed secretly sabotage the construction plans, whilst of course still putting food on the table (needs must)."
"As long as/Provided you don't eat your vegetables, you can't/won't be allowed by your mother to play outside. So I suggest you continue to not eat them, seeing how you hate playing outside. But don't tell your mother I said any of this!".
Again though, I myself sense a blurring of meanings somewhat.