Dear Molly, 

Q1. When you are performing a meta-regression for one group the results _1 correspond to the minimum value of the group. _1 correspond to the maximum value of the group, and _1m0 would be the slope, the difference between the minimum and maximum. So you are right, in your interpretation.

Q2. Yes indeeed you can perform meta-regression with categorical variables. It is a good way to analyze it.

Kind regards, 
Lydia