To answer your questions:
1. For NIH Phase I and Phase II SBIR projects, the contact PI/PD needs to spend at least 50% of the time in the small business at the time of award and during the conduct of the proposed project. Deviations from this requirement are rare and must be approved in writing by the grants management officer after consultation with the NIH SBIR/STTR Program Coordinator.
For Phase I and Phase II STTR projects, the PD/PI is not required to be employed by the small business. However, the Contact PD/PI, the first PD/PI listed, must have a formal appointment with, or commitment to, the small business, which must be in the form of an official relationship between the parties, but need not include a salary or other form of remuneration. Each PD/PI on a multiple PD/PI award must commit a minimum of 1.2 calendar months (10% effort) to the project.
There is no time effort requirement for consultants.
2. When the small business cannot finish the proposed work by itself and need collaboration and intellectual input from other institutions, subawardee is usually needed. All subawards will need subaward PIs.
3. Yes, you do need letters of support from consultants to show their commitment to the small business.
Feel free to reach out again for additional questions!