Even if you have worked in the industry, unless you have done so specifically for Zavvi or one if the other steelbook retailers then it's entirely speculation. Saying that Zavvi HAVE to take certain art seems unlikely when we can all see that the same film gets released in multiple countries with different art, extra discs, lenticular sleeves, postcards etc. I'm not going to debate the merits of those things as I know they are worthless to some people - but Zavvi could choose to add these things if they wanted to.
Likewise if they are given a take it or leave it policy to steels then how are much smaller companies able to get the same studios to sign off on unique releases? Kimchi is basically a one man band (not holding them up as a perfect retailer here - just an example) and yet they produce good products with different art and sleeves etc. you're saying they have more sway than Zavvi? Seems unlikely.
Plus the stock levels of the product is less up for negotiation with steelbooks. As it's common knowledge that a 4k print run is the minimum, and when companies sell less than that the same copies seem to turn up at another retailer abroad. If anything the shift by Zavvi to these 2k runs shows that they must have some sway - as a lot of the 4k runs weren't shifting until heavily discounted so now they offer lower quantities at a higher price to drive demand. That would have been Zavvis call (based on how their steels to date have performed) rather than the studios. Would every studio simultaineously tell Zavvi to only stock 2k copies (which is what has happened recently) seems unlikely.
I suspect that Zavvi could argue against the art choices more often than they do, but I've no doubt their hands are tied in some cases, but they still have the final say about whether to sell any product - as it's not like they have much uk based competition when it comes to steelbooks.