Hi Fluff's mum,
I am so sorry about the diagnoses. You probably feel like a ton of bricks just fell on you.
I don't have any direct experience with hyperT or heart murmurs, but the combination of hyperT, heart murmur, and CKD isn't unusual, and all three conditions can be managed for quite some time. Not all heart murmurs are significant, and sometimes they come and go. Did the vet feel it was enough of a concern to refer you to a cardiologist? Twice-daily subQ sessions is unusual; is that due to a fear of potentially overloading the heart?
As for the hyperT, I assume that Fluff is getting methimazole.
CKD, unfortunately, I am very familiar with, having lost my Celia to it in April. It is absolutely true that CKD kitties can live a very long time, though of course there are a lot of factors involved (stage of CKD, treatments, concurrent conditions...). HyperT can mask renal disease and make CKD look less advanced than it is, but even so, that does not mean that you cannot still have a lot of good time left with Fluff.
One of the most important things you can do for Fluff is to learn as much as you can. Tanya's Comprehensive Guide on CKD is really the go-to; it has the blessing of numerous vets and other organizations: Tanya's Comprehensive Guide to Feline Chronic Kidney Disease - Everything You Need to Know to Help Your Cat
. What I learned there and from members of its affiliated support group was invaluable and gave me 3.5 more months with Celia after the vet recommended euthanasia.
I would also strongly encourage you to keep your own copies of all labwork results. Vets don't provide them as a matter of routine, but all you need to do is ask. Just let them know that you want to be able to track Fluff's progress yourself at home. If you have them and would like to post, I'd be happy to take a look.
When you say Fluff seems depressed, is he lethargic? not eating well? Since it doesn't appear that your vet gave you a doom and gloom scenario, any symptoms that Fluff is showing are treatable with the right medications and supplements, so don't be discouraged!