AEAD: Is it secure to decrypt the content encryption key before verifying the MAC?
No, RSA encryption does not provide authenticity, regardless of the padding. This is for the simple reason that anybody can encrypt with the public key. This is irrespective of the asymmetric algorithm used. You need to sign the data to obtain authenticity.

What AEAD does do is to protect e.g. against padding oracle attacks. These can be performed on CBC mode encryption, which could also cost you confidentiality (leaving you with nothing).

In that case however the RSA implementation must also be protected against padding oracle attacks. That basically means that OAEP (or possibly RSA-KEM) should be used instead of PKCS#1 v1.5 compatible padding. Otherwise the PKCS#1 v1.5 unpadding must be performed in such a way that padding oracle attacks are impossible.



With regards to this specific protocol it seems that signatures are handled by a separate document: JSON Web Signature (JWS). Section 11.4 is called "11.4.  Adaptive Chosen-Ciphertext Attacks" and explains the attack on PKCS#1 described above.