I'm glad to hear the garage want to replace the plugs, assuming their diagnosis of a dead cat is correct, the most common cause is overheating, usually due to unburned fuel entering the cat - this usually comes from defective plugs not firing the mixture correctly.
With this in mind i would strongly advise an oil/filter change while it's in there, i know this will cost a bit more but will help keep the VVT mechs functioning as they should, a sticky solenoid on the bank where the cat has died could also be a possible cause as the valves would be opening more on the other bank to admit more fuel/air but if the valves didn't open as they should, the extra fuel would be wasted and sent out into the cat.
Also the much more basic reason of possible fuel contamination of the oil.
Given the misfires and the fact one cat is dead, i would suggest new sensors, both banks, pre and post cat so 4 sensors in all. I know this is going to hit you in the wallet area quite painfully but this is where the old adage of "A stitch in time saves nine" really applies to modern day stuff. In other words, if the problem had been investigated and fixed back then, the chances are that there wouldn't be a collection of compounded problems now.
As far as the Lambda sensors go, they have a working life of ~100k miles or 10 years, in practice they normally reach 120k miles without problems but their performance drops considerably as a result. In some parts of the USA, notably California (and some other states but can't remember which) a mandatory fitment at build is an "Emissions Timer". This runs for 10 years or 100k miles and then puts a fault onto the engine management system. It can only be removed by fitting new Lambda sensors and emissions timer and then resetting the EML.
Fingers crossed they can find a cost effective way of doing this all for you!