Q1: Yes, you should specify a bead size. I have no experience with ceramic bead so I cannot advise on a size; however, you may not be able to achieve 12-16A on 15-5PH without a lot of media fracture.
Q2: Yes, intensities selected are OK.
Q3: Anything close to 100% coverage will be adequate to your purpose.
Q4: Not only will the stress relief bake at 375 (step 6) not affect the shot peening, it will affect nothing at all and I recommend you eliminate it. You should also eliminate the stress relief bake in step 1. Not so the hydrogen embrittlement relief in step 4. It is necessary for the purpose of hydrogen content reduction and it will not affect the shot peening residual stresses.
Q5: Either AMS 2430 or AM-S-13165 is appropriate to your use in this matter.
