Having Threepio, Artoo and Tarkin as real characters makes a lot of sense to me.

Having Artoo spend quality time with Ben seems a bit odd but Ben isn't exactly the most truthful character in the saga (from a certain point of view) but having Threepio built by Vader and hanging around with Owen (for months if not years) is utter nonsense.

Having Tarkin as Scorpius in a different Halloween mask for two seconds makes no sense at all.

I'm not sure if we needed to see Yoda he could have just have been referred to like Jabba in the OUT and unseen until ESB thus maintaining the surprise.