Attic was built to be living space in 1945, but are the trusses strong enough to keep doing so

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.

The lower vents will indeed be less effective than the upper ones. In addition, you are reducing the effective insulation of the wall/roof in order to install baffles. With effective cross flow ventilation and a proper ceiling vapor barrier, you only need a total of 1/300 of the attic area in open free area ventilation. IMO, getting at least 1/2 this minimum area from the soffit is the important part. Actually balancing the areas when you have more than adequate ventilation is less important.

If you can achieve this 1/2 minimum area in the upper soffits alone, then forget about the lower ones all together. Only if you have no choice would I attempt to bring in air from the lower soffit. I'm fairly dubious on just how effective these would be.