Trying to Determine if Attic Braces and Collar Ties are Structural

Yes. Very possible. It has to be engineered though. We have plans to do just this. The drawings and engineering cost around $1000. I wouldn't try it unless an engineer has looked at it, drawn a plan and signed off on it.

For us we are going over a 3 car garage and the roof pitch is 12-12. So a lot of space there. The plan is to remove the drywall ceiling in the garage and install 24" engineered joists (2x4's). We lose a little head room in the garage and the space above, but not much.

Then once the floor system is in place it's much easier to move around. 2x6 rafters installed with collar ties at about 8.5 feet, and plywood gussets with engineered nailing patterns at the peaks. Then cut away the trusses. Doing it for about $80 a foot, and it will add about 500 square feet.

You don't have joists or rafters. You have engineered roof trusses. The bottom chords are 2x4 because that's all that's required for your scenario. They don't span the ceiling themselves. They're part of a rigid structure that's supported by both the top chords and the diagonal truss members. This is typical and has been standard practice since the 1970s with virtually no static failures on record.

Some trusses, in the case of very large spans or girders (which are sometimes doubled or tripled), are designed with 2x6 or 2x8 bottom chords. But for common trusses in modestly-sized homes 2x4 is standard.

The blocking was probably added to support a particular ceiling finish--tiles or wood panels, for example. Unless you have a particular need for more weight-bearing capacity, leave things alone. If you do, consult a licensed engineer who will examine things much more closely than we're able to from one shady photo.