question about my current routine

hey guys, I've got two questions about my current high volume routine. I'm basically doing an upper/lower split 4 days a week, and am trying to keep the rep range around 10-12. The thing is I'm finding that I'm struggling to make progress with my standing barbell curl at 3 sets of 12 reps, but my incline dumbbell curl is coming along fine at 2 sets of 12 reps. The questions are as follows:
 
Would doing heavy sets with a low rep range of 4-6 with the bb curl, and then high volume with the incline db curl be better to make growth and progress if I'm currently stalling? Is that considered undulating?  ALSO

For triceps I'm currently doing lying tricep extensions and seated dumbbell extensions, do they work the same part of the tricep?

For triceps I like to add kickbacks because resistance is maximal at full extension (w/nice squeeze!)where I believe it is lowest on the extensions.