Wow, thank you both very much for so much good information! I am feeling much relieved having seen that E Extra seat map.

Yes, getting pet-in-cabin approved was quite an ordeal. My advice to anyone undertaking this is to strictly avoid any code-share scenario. The differences between airline IT systems, processes, and policies are very difficult to overcome.

Well, I will report back with my experience. Once again, thank you so much for your thoughtful responses!