It would be tough to answer your first question because ACNP's with RNFA certification probably represent a small number of the total population of ACNP's. In addition, ACNP's represent a smaller fraction of the total number of NP's (there are states that do not even have a single ACNP program). Lastly, not all NP's who are also RNFA's are ACNP's, there are some who are trained as FNP's or ANP's.
I think the ACNP track attracts nurses who are interested in hospital-based roles particularly ICU's and other high acuity specialties such as Cardiology. Surgical specialties are constantly present in the hospital, thus, this is also a favorite practice setting for ACNP's. When you say Doctor-in-a-box, are you referring to retail clinics? That's not an appropriate setting for ACNP's.
The general rule of thumb is that NP's find jobs in fields that match their experience, NP training, and other special skills they may have (ie, RNFA). Having the ACNP/RNFA combo is attractive to employers
in surgical specialties in job markets that have a need. It is possible that UAB us not able to give figures because their graduates do not stay to practice in the Birmingham are after graduation.
Surgical specialties typically generate better revenues than medicine-based practices that are not heavy with procedures. The colleagues I knew were making six figure salaries in the Midwest as new grads. However, six figures are not unusual for any ACNP where I'm at now even for those who don't have RNFA like myself. I'm sorry I can't really give you concrete answers. Just like in anything else, there isn't a crystal ball that tells us how our careers would go in the future.