Calling USCIS will not be useful. They will just tell you that it all depends on the ASC you are supposed to go to. USCIS has given the responsibility of taking fingerprints and pictures to local support centers in order to increase efficiency and to allow people to be able to get to a place without travelling to a new city.

The specific rules of whether walk-in or not is ASC location dependent. Some places allow walk-ins, others don't. Some places only take walk-ins on certains days or at certains times. You will have to make a trip to your ASC to find out what their policy is.

Walk-in's will not "mess" or help your process. Your process will just proceed as is. Even if you get your biometrics done early, your case worker has their own timeline on when they will work on your case. The 60 - 90 days from the time you turn in your application is the only consistent thing told and it is what you will hear when you call USCIS and from people who have had to deal with USCIS. Now the only difference is whether 60-90 days is just for 821D approval or for 821D approval + I-765 approval. No one knows this answer, but we will find out the timeline for us when we see people reporting what they receive.