Hi there. I'm so sorry to hear about your cat Haggis. I went through a similar thing with my cat Tiger who had Squamous Cell Carcinoma as well. He had a tumour in his nasal passage. I learned about Transfer Factor (a super immune booster) through Catty1 and emailed back and forth with a woman who had given it to her cat who also had S.C.C. Her cat was in very bad condition and she didn't want to put him through surgery. She scheduled his euthanasia but just couldn't go through with it. She started him on Transfer Factor and within months he was greatly improved until he was in remission. Her vet was very surprised and her cat is still with her today, years later.
I bought some and was able to give it sporadically to my cat Tiger but he was so fussy that he just wouldn't eat it if mixed in with other food. My other cats couldn't even tell it was in there because they would come up and try and eat his food but Tiger wasn't having any of it. I didn't want to force it on him because whenever I did that it was very stressful for him. I ultimately had to let him go to the Bridge after a year but I really think that if I had been able to get the Transfer Factor in him, he would have had a much better chance.
I have half a bottle left over and a whole bottle that has been compounded into a thick chicken flavoured suspension by the compounding pharmacy if you are interested. I would have to check with them first to see if it is still okay to use as it was compounded a few months ago. I have also offered this to KittyKatherine, whose cat is also fighting cancer. If she would like some too, I could send one of you the bottle and one of you the suspension.

Whatever you decide, my thoughts are with you.