Hello,
I think that I would ask for a second opinion from a different vet at another practice. I say this because I cannot interpret lab values without an examination. These go hand in hand to make an accurate assessment of a patient. Ask your friends and family who they use and trust and go there.
I also think you should bring all of the things that you mention above to your old vet.
Also I think it is important to talk to your vet about your financial abilities and ask for diagnostics you can afford. This is your right and the vet’s responsibility in my opinion. If they cannot help you find answers that are meaningful to your pet and their care go elsewhere.
Lastly I have been a vet for almost 20 years. I have not had many patients able to do an mri. And although I may offer we always (always!!!) tell about more affordable and readily accessible diagnostics like an ultrasound. I also ALWAYS talk about why I am asking questions them to do this. I fear very few vets do this. There is almost no point in asking you to do (and pay for) a diagnostic if we cannot do anything about the disease or problem from there. So what is an mri going to get you that is treatable? And Would you be able to afford that? If the answer is no why are we worried about the mri?
For cats with constipation issues I recommend a high quality canned food only diet and lots of exercise. If you are worried about pain please talk to your vet about safe options for cats. There are not a lot of them.
Thank you. Since we’re dealing with several other health issues my cat has been less mobile, which I know was part of the problem.