My advice would be not to worry so much about NSI or the NROTC unit in general. From what I am hearing NSI is going to be mandatory going forward and current 4/c mids who did not attend last year are being sent this year. Based on this, I would think most college programmers who apply to their units reasonably early should be able to attend. Even if you were not able to attend this summer, you would be fine. You would be able to make up the knowledge during your unit's orientation period. I highly doubt if any unit would be able to assure you that they could send you to NSI at this point.
The fit of the school is much more important. You will spend most of your time as a college student and this is much more important. The leadership and participants in an NROTC unit change often. My DS is in his 2/c year and none of the unit staff that was there when he was a 4/c is still there. Each new leader brings in their own experiences, priorities and personality and can have an effect on the operations of the unit. The NROTC curriculum however is the same at every unit. The culture and personality of a school changes much more slowly. Find the school that you will be most successful at.
If you do know where you plan to attend, there is no harm and may be beneficial to reach out to the NROTC unit to let them know you will be attending in the fall, you are still waiting on a NROTC scholarship decision, but are committed to joining the unit as a college programmer if you do not get a scholarship and ask if there is anything you should do now, such as applying as a college programmer.