Oh, for heaven's sake!!!!
How about going in to the centre, asking to speak to the Manager/ess and requesting (politely, of course) that :
* someone show you how to do the meds so that you can take these cats home to a dedicated, warm, environment where they can get one-on-one attention and not have the stress of other cats and lots of people around them.
*Acknowledge that the move may not be ideal in the first instance but compared to living in a small pen with the busy bustle of the cattery around them, it will be nothing.
*Make a point of reminding them of your cat owning history, play up the fact that you have shown your loyalty and dedication to these little cats by being such a frequent visitor (therefore minimising the impact of taking them home)
* Let them know that you are frustrated that you are attempting to do the right thing by these cats, have provided them with all the (insane) information that they have requested, and point out that you taking them will free-up the pen for other needy cats. (rescue centres always have a waiting list of cats to be rehomed, don't they?!)
* Tell them again that you are happy to take the cats to YOUR vet, pointing out that this will save THEM money.
Remember: as hard as it is, remain calm and don't get cross (not that I am suggesting you will be but many people by this stage will be tempted to shout in frustration!).
And if all the above doesn't work, cry!