Both of yours scenarios work, via person B doing a different commissisoning program than STA-21.
A nuke is not considered a nuke until fully qualified (aka. after a year and a half of training), so students do have a better chance of getting picked up for these type of programs.
Nuke Officers are also needed through these programs, and enlisted nukes are a good procurement, especially since they have some experience/training related to the job.
You can apply for something else, but usually your chances of getting it are lower.
For actual data, go to
www.npc.navy.mil look under the references tab, then go to NAVADMIN's. Search for a message like Seaman to Admiral results. You can see the past few years through this to get a trend. Keep in mind there are non-nukes who get selected in these programs too. There is no set number of people who get accepted each year, as the selection rate varies with manning requirements.
Jason