Rang, sending you lots and lots of hugs. I lost my soul kitty when he was two. Officially he died of Hepatic Lipidosis, but that was secondary to some unknown primary illness. I feel your pain and it is heartbreaking. I am so sorry you are having to make these difficult decisions.
Personally, I too would not do the chemo either.
I do have a question for you about the exploratory surgery though. Is the reason for the exploratory solely for identifying Lymphoma or does your vet believe there is a good possibility he/she will be able to find an alternative problem?
Has your vet ruled out hyperparathyroidism? This condition does occur in cats and will also cause elevated serum calcium. If this hasn't been ruled out, please do this before you make any final decisions.http://www.peteducation.com/article.cfm?articleid=298
I do know though, that extremely elevated serum calcium is considered a tumor marker in cats so I understand you fears.
Please remember to take good care of yourself too during this horribly stressful time I will be sending lots of vibes for a postive outcome.