Question regarding where you're having trouble
Are you having trouble getting the weight itself up? Like there is a sticking point.
Or are you getting the weight up but you can't hold it after you attempt to lock it out?
If it's the former... then yes ME work on the basic "grind it out" exercises are necessary and DE work will teach you how to blast through the plateaus. With your DE work for this movement, it may be advisable for you to begin with a bit more weight.
I'm not sure other's might want to chime in on this. I"d reccomend starting with 65% instead of 50% for week one simply because of the nature of the exercise.
I'm not exactly sure though...
If it's the latter, then nothing really helps other than practicing the lock out itself (set up high barbell in power rack, lock out with 1 hand and 2 hands). Also work the supporting muscles needed in the movement.
I believe if there is unstability it will originate from the joint regions... so elbow and shoulder.
More than likely weakness in the elbow will stimulate from weak forearm muscles and tricep muscles... perhaps also the bicep (but most tend to train this anyway)... identify your weakness and train it to be a strength.
In the shoulder: most likely the rear delt will destablize the lift if not properly trained.
K, its 6:30 and I"m jsut getting to bed...and i'm losing my train of thought...so if i'm forgetting something... i'm sorry good luck