I would use the chance to replace them all and make sure to check your spare. Then keep the good tires and sell them on crlst. I did that and ended up with the tires I wanted for way cheap minus the sell of the old tires. Then your ready to hit the road or trail for a while.
No, No, No. replace all 4, and if you have changed from the stock size replace the spare and add it into your rotation mix. The 100 is full time AWD and tires should be within 4/32 max of eachother. I'd prefer 2/32