a collar tie, from my understanding, is a cross tie near the peak of the roof. It is to keep the rafters together at the peak during periods of uplift. They would not provide adequate restraint to tie the walls together to prevent spread as the leverage applied would easily overpower their ability to restrain the rafters.
the boards you are dealing with would simply be joists as far as I understand.
They are intended to prevent the spread of the rafters which are attached to the walls so both would end up spreading. I cannot tell you which is the most proper method of attaching but since the rafters are attached to the top plate of the wall, to me, it becomes a moot point.
to the ties on the trusses; not what you are looking for.
What you're suggesting is a decorative truss, correct?
no. just suggesting you can dress up the joists that are present.