There is no “crystal clear” evidence. Unless Han explicitely states in the movie “I am part of the rebel alliance”, there is room for interpretation. Just accept that other people might interpret the same thing in another way, instead of keeping up this fruitless discussion. At this point, all arguments have been made for and against it.
Even if Han explicitly said “I am part of the rebel alliance” you could still claim he lied.
Now please wake up and fasten your seatbelt as we are about to land in Reality.