#1) Fabricators test in the 2G position usually because everything can be turned (by hand or with a crane) and welded in either 1G/1F or 2G/2F positions, the 2G test qualified a welder for all of these positions.
#2) The reason the coupon slips off center while bending is the single bevel, as the coupon is stretching around the die it favors more to one side due to the asymmetric shape.
#3) With 2G, I had lots of students struggle with the cover passes drooping and rolling over once they were out of the groove and there was nothing to help hold the puddle up. I had them practice running stringers across a flat bar that was held in the same position as the coupon. We welded a 1/2" thick plate to the work table, drew a line across the plate with soapstone and they ran passes across the plate and practiced stacking the beads(basically padding the plate) until they could run caps/cover passes with confidence. When you don't have the ledge to lay on, you have to move your hands a bit quicker as the plate starts heating up and the puddle wants to droop down.
Lawrence has good advice about utilizing stringers without whipping forward/back or any side to side weaving, all that does is add heat. These types of puddle manipulations are good to have in your welder's tool box for a few instances where fit-up or material thicknesses that aren't ideal for the diameter of electrode that is being used, but for testing, they need to leave them in the toolbox and use straight smooth stringers for more consistent results.
Edit* and yes use run on/off tabs, that is a good practice to get into as a new welder testing for a fabricator. When splicing a flange, the run on/off tabs ensure the flange thickness is maintained across the entire width of the flange.