I've been thinking more about your suggestion to look at conservation of
momentum as a way to introduce momentum before force and I like it. When
looking at video analysis data for collisions between objects of different
masses, it would be instructional to show that during the collision:
1. delta(v1) is not = - delta(v2)
2. a1 is not = - a2
Then introduce momentum and show that:
3. delta(p1) = - delta(p2)
4. delta(p1)/delta(t) = - delta(p2)/delta(t)
This would show the significance of momentum and might be an experimental
"justification" as to why momentum is used in physics. Force could then be
defined and Newton's third law experimentally verified.