Hi @Anne,
Momentum is always conserved. The only question is whether we consider other forces, such as friction, diverting the momentum elsewhere.
For your L2 exam, the statement “Momentum is conserved in the absence of external forces” would be a common expectation. This means that momentum is conserved in two rollerskaters who collide with each other (sum of momentum before = sum of momentum after) as long as we ignore any frictional forces acting between their skates and the ground. If we consider friction, then some of their momentum will be transferred into the ground, so their combined momentum after the collision could be less. Note, in this example momentum is still conserved, but we may not know how much has been transferred to the ground.
For your second question, in torque or bridge problems you identify a pivot as your reference point. Any forces acting through the pivot are at zero distance from the pivot, therefore contribute zero torque.
This post might help you.
This post might also be useful.
Let me know if you would like further clarification.