Is the decompression of compressed and encrypted data without decryption also theoretically impossible?
The decompression of compressed-then-encrypted data is not possible without the decryption key, at least for compression and encryption schemes independent of each other. We can make a theoretical argument for that: compression schemes compress only a small portion of possible plaintexts (that happen to be the ones where compression is used in practice), and slightly expand the others (e.g. random data). Encryption makes it impossible to distinguish if two ciphertexts of equal size correspond to plaintext that compression compressed, or not; thus prevents any meaningful decompression.

In the question's situation, the classical solution (described as "Another option" in the question) is to compress at A, then encrypt, then transfer the compressed+encrypted data to B. On retrieval, that same compressed+encrypted data is forwarded from B to A (or A'), decrypted, then decompressed. This reduce bandwidth for backup and retrieval, and storage requirements at B.

B does not need to decompress or decrypt the data at any point: the question's "but then B should be able to decrypt the data to decompress it" is true, but a non-issue. B does not need the key, and (thus) can't decrypt.

What is an issue is direct access to a portion of a huge data set: for many common compression algorithms, that requires transfer of all the data before the point being accessed (sometime, all the data). This is solved by a split-then-compress-then-encrypt strategy, where the plaintext is split in segments that are compressed independently, then enciphered independently (or mostly so). But the splitting tends to reduces compression, especially for short segments.

Another issue is that the compression ratio leaks to an eavesdropper, and that reveals something about the data. This can be a serious issue: for voice, it has been shown to be enough to understand what's being said!