Washington is an interesting case, he isn't even a first choice spinner in his domestic team and only got in the test team when he was part of the India tour of Australia, he was a bowling reserve for practice only and never meant to debut there. His batting is a big plus but it won't matter as long as you are not improving with the ball and so far, he didn't show any positive sign.
I guess, they will have to replace Ashwin with 1 batting-spin and 1 spin batting allrounder and hope they chip in together and bring at least 7-8 wickets and contribute with the bat.
Those domestic players are above 33 years means they won't be considered in my opinion and Washington Sundar is the only available off spin in Ind there's even anyone yet in India A team so as you said if he can tweak his skills he definitely got a big opportunity cause he got the batting ability which proven already so when there's the right time I think he should be given a fair run to decide whether he is capable of making it or then we have to go with left arm and leg spin for a certain period of time.