Are そういう and そんな completely interchangeable? Is there any difference in nuance or meaning, or contexts where one is more appropriate than the other?
Most of the time they are interchangeable because their meanings are almost identical. The only notable difference is that in cases where the speaker is expressing negative feelings and wants to emphasize them, 「そんな」 is more appropriate, because it is more emphatic than 「そういう」. For example:
In those sentences, if you replaced そんな with そういう, they would be less emphatic. Of course, sometimes this might not be true because you can make them sound aggressive purely through intonation, however, it wouldn't "ring" the same.