It's correct except that the comma should be replaced with a semi-colon, dash, or full stop. The clauses with "was" both refer to the same past, finished time - when he or she saw the tree. The clause with "have ever seen" refers to the whole of his life, up to the present.

Technically the sentence should be in the Simple Present: "That IS the tallest tree I have ever seen." This is a fact that was true in the past and is still true in the present, a situation which is best described by the Simple Present verb tense. However, this is pretty pedantic. Most people will say "That was the tallest tree I have ever seen" if they are referring to a tree in a story they have just told and the action of seeing the tree occurred in the past. The biggest problem here is that the sentence is actually a run on. It should be punctuated this way: "That is/was the tallest tree I have ever seen. It was amazing."

Yes, that sentence is correct. You would use this structure in an everyday conversation. Present Perfect is being used because from the moment you saw the tree until now it's still the tallest you've ever seen.