Technical feasibility of decrypting https by replacing the computer's PRNG
1 - How feasible is it that the chip's manufacturer can predict the output of this PRNG when it passed tests from the people applying the use of this RdRand instruction in kernels?


A strong stream cipher's output is random and unpredictable to anyone not knowing the key. See where this is heading? Just because something looks random doesn't mean it's random.


  2 - If the chip's manufacturer can predict the output of the PRNG to some extent, how feasible is it that they can decrypt any https traffic between two systems using their chips? (Or anything else requiring randomness, https is only an example.)


If you can predict the PRNG you can basically predict the secrets used for key exchange, and from that deduce the shared secret. Then you can simply decrypt the communication.