1) Yes. Thin rubber is slightly permeable to air molecules so they will gradually lose pressure. There's nothing wrong with the tubes.
2) Best is to just pump them up regularly. You can get heavy-duty tubes made of thicker rubber but they'll be heavier and also (separately) have greater rolling resistance due to the hysteresis losses from flexing the thicker tube as the wheel rotates. Adding Slime is an even heavier way of reducing the rate of air loss. There are Presta valves that are removable to let you add the Slime. The other method is to cut a hole in the tube, add the Slime, and then patch the hole.
3) Best to ask them instead of us.