Question

If a person is not already in custody, the police must have what justification in order to "seize" that person for compelled participation in the lineup?

Answer 1

I believe the answer is: reasonable suspicion

Investigation under reasonable suspicion would give negative backlash if proven to be abused by the officers, but it still give them legal stand point to conduct a search on private properties.
Officers need to find things such as blood evidence, screams, or gun shots to conduct searches under reasonable suspicion.