With INZ confirming that the partner can get an interim visa with rights but when the time comes that interim visa is only visitor, can we appeal it to INZ?
I don't think so. The INZ telephone answerers are not case-workers. They are a human FAQ system, working from a database, and as you know, what anyone gets out of a computer search depends on what key-words they put in, how readily they realize what is or is not relevant, how persistent they are, etc., and I'm afraid that it's become apparent down the years that it's perfectly possible to get wrong answers from them (though delivered in all sincerity, I'm sure) - even three different answers to the same question on the same day.
All processing times are unpredictable at the moment. If you do a search on those kinds of visa on the forum here, you'll find what has been happening to other people.