Nothing personal -- purely academic curiosity. Grammar is by no means my strong point.
I believe that you are correct in declaring 1a and 2a equivalent because of the "an" -- my interpretation ignores the "an" in both of the original sentences and so is incorrect (but, I believe would be the correct interpretation if the "an"s were not there).
The problem I have with the way 2b is parsed is the concept of [beauty which serves no practical purpose] as a standalone concept. [Beauty that serves no practical purpose] makes sense to me, but "which"? The interpretation in 2b seems to imply an ill fitting afterthought about the purpose of beauty (which, incidentally, serves no purpose) that has little bearing on the intent of the sentence. Because of this I think that 2a is the only reasonable way to interpret the second sentence.
I could easily be wrong again though....
1a. There is [an intrinsic value in beauty] that serves no practical purpose.
1b. There is an intrinsic value in [beauty that serves no practical purpose].
2a. There is also [an intrinsic value in beauty] which serves no practical purpose.
2b. There is also an intrinsic value in [beauty which serves no practical purpose].
All of these interpretations work for me. To paraphrase:
"There is an intrinsic value in beauty, and this value serves no practical purpose." = 1a and 2a.
"Beauty serving no practical purpose, this kind of beauty has an intrinsic value." = 1b and 2b.
Yes -- I see, although more accurately:
"There are intrinsic values in beauty, and one of these values serves no practical purpose." = 1a
"The instrinsic value in beauty serves no practical purpose" = 2a
"Some beauty serves no practical purpose -- this type of beauty has an intrinsic value" = 1b
I'm not sure 2b is a valid interpretation.
I think that the use of "an" in both 1a. and 2a. restricts each of them to "one value", no? Both "which" and "that" are perfectly compatible with plural referents, as far as I can tell. Also, what is it about 2b that makes you unsure?
By the way (before another heated personal debate starts), I pose these questions honestly. I think that everyone has valuable linguistic judgements to contribute towards an interpretation.