My #@%$ Cat Destroyed My ARC REF 5SE. Soliciting Suggestions.

Cat pee corroded the board?  Sounds like it may have taken some time and several occurrences.  Didn't you smell it, like the first time it happened?  Cats can make reasonably good tacos, and maybe a pair of gloves, once skinned.  When you replace the amp, maybe put it in a cabinet. 

Ii was not your cat!!!!!  Door was open to your house- a bird came in as well as a cat. Do the math-=-=

oh man, sorry to hear. This was always a worry of mine when I had  big Levinson amp on the floor that all my cats sought for warmth. My new integrated fortunately fits on shelf with only an inch of room on top so they can no longer access. Glad to see you are a good, considerate cat dad and did not take it out on the poor scared kitty.
if the new board includes warranty and testing once installed, I would do it.

Very sorry for your situation. I agree 100% with teo_audio. I've repaired amps, preamps, crossovers and other gear with printed circuit boards, built circuit boards, amps, preamps, and more. I've rebuilt traces, cleaned boards, repaired burned board sections, and more. I would be very hesitant to declare it irreparable without actually seeing the damage, though I would not pay $6k for the board.

Would you be OK with posting pics? You can PM me with them if you wish. I won't do the work, but could possibly offer another opinion that aligns with teo_audio, which might encourage you to seek out a willing tech. The cost is substantial, so I encourage you to not give up quite yet.

Also, although you didn't solicit veterinary advice, anzaanimalclinic provided excellent information.

I haven't seen it mentioned on this thread but cats definitely do not like being left alone for extended periods. Maybe the delay you encountered contributed to the cats behaviour
I was once told of a couple that went on vacation for two weeks (and presumably had a "cat sitter' for the duration)
When they returned from vacation the cat immediately jumped on their antique cabinet, looked them in the eye and proceed to pee all over it