“I wonder if we can think along the same way here: there is no “san” in indriya, but “san” is involved in ayatana.”
– Almost right! A little bit better to say: “there is no “san” involved when using sensory faculties as indriya, but “san” is involved when they are used as ayatana.”
“However, if this is the case, then I remember reading about nibbana some time ago, in a post where nibbana was referred to as tadayatana. I wonder why ayatana was used there in the context of nibbana; “tadayatana” sounds like a contradiction in terms.
– The post you refer to is: “Nibbāna “Exists”, but Not in This World”
– The word “āyatana” is used there for the lack of a term to describe Nibbana. None of our words are applicable and “sphere” or “dimension” may be the closest that we can use. It has nothing to do with the 6 six sense faculties. I have revised the #4 of that post to make it a bit more clear.