With two people, I wouldn't worry about about bears. Ditch the mace, my friend. I had this conversation with the park ranger, if the shelter is full you are allow to hang around it. However, if it isn't you are suppose to sleep on the bunk unless you're alone.. The Smokies temperature will vary at high elevation during that time period, I would plan for at least 20 degrees at 6,000 feet and above in late March and April. I might see you somewhere along the way!!!
Edit: I stayed at Tri-Corner Knob shelter in late July and it was in the 50s or slight lower. So, I can imagine it be colder in late March and early April..