Just catching up on overnight posts Anthony... you've had some great advice here! Def worth adding a little water to Shelley's food to keep her hydrated. Cats do have a low thirst drive so her "normal" level of water intake mainly indicates that her diabetes is not under control. At times when her numbers are lower, she may not drink quite as much... I've had a look at her ss a couple of times lately and she does seem to be getting some decent blues during the cycle. It's the pre-shot numbers that still need to come down...I know we have debated this, and dose changes before, with mixed success!
Particularly given Shelley's age, supplements of some kind are almost certainly going to be needed... Cosequin or B12 as you decide, again I think you have considered this before. I know you're reluctant to go against the vet's advice and this is always a dilemma when people here recommend certain things or courses of action. It's a case of weighing up what you can research and making a considered decision. The vet is correct however about being cautious about prescribing Metacam, it may be harmful especially if used on an ongoing basis. But again, you have to weigh things up - I know an extremely experienced member here who gives her elderly cat Metacam because it does the job with no apparent effects, and she wants the kitty to have the better, pain-free quality of life it provides.
Sorry not to be much practical help but those are my thoughts FWIW. Let us know if Shelley starts drinking again today, add a little water to her food and consider a supplement. If she only saw the vet a couple of days ago there is unlikely to be any sudden change in Shelley's overall health to pursue any further course of action at the moment... if it's as hot where you are as it is here in the south, I still say that my original guess about the heat causing lethargy might be a factor.
Good luck and keep us posted.