Can methylisothiazolinone in wet wipes be the cause of anal itching and skin irritation in that area?

yes if u r allergic. Isothiazolinone is a preservative which can be an allergic contact sensitizer. It is on virtually every patch test kit dermatologist use. If you want to see if you are allergic, wipe some on your left inner arm, occlude with a band aid x 48 hrs. Remove it and look in another 48hrs to see if you have a raised red rash. If you do, you've made the diagnosis!
Don't know. One of my colleagues might know more, but I am not familiar with this ingredient and its potential to cause your symptoms. That said, any substance can be the source of allergic reactions. Just using a wet wipe (if the area stays wet) could foster surface infections (e.g., fungal). Consider a trial of avoiding the chemical to see if things improve.