Dear David,
I’m not sure I can comment on your input file settings, as these will depend on the conditions you want to linearize about.
While the linearization within FAST v8 is powerful, one limitation relative to the linearization functionality of FAST v7 is that only the first-order form is calculated by FAST v8 (in FAST v7, both first- and second-order forms could be calculated); that is, in FAST v8, it not possible to directly obtain the linearized mass matrix. Of course, in first-order form, what you do get as output is -[ M^-1]*K and -[ M^-1 ]*C in the linear state matrix (A).
Best regards,