Question

an isolated system of two particles are released from finite separation. there are no other forces on the particles except internal force. in any hypothetical situation
a) is the internal force are equal in magnitudes opposite in direction but not along joining them the why mechanical energy is not conserved?
b) if the internal forces are not equal in magnitude but opposite in direction and along the line of joining them then mechanical energy not conserved, linear momentum not conserved and angular momentum about only some special points at any time is conserved?
WHY?