I think the tri pushdown in good form requires you to:
pin your elbows to your ribs- in order to minimise assistance from your shoulders and traps.
Leaning forward excessively may also allow your delts and traps to assist in the exercise. Keeping your upper body upright but having one leg a half step behind your leading leg will help with balance. Alternate the leading leg with different sets.
However if your elbows actually hurt, think seriously about changing the bar with which you do the pushdown- the rope may help your elbows too (you'll probably have to reassess your weights as using the rope is harder- for me anyway. Great exercise tough and I'd use the rope over a bar).
What causes you pain now will not go away if you don't change something- especially as you increase weight with HST.
As far as range of motion goes, I'd go further up from the 'L' that you describe but ensure that my elbows don't move from their position next to my ribs.
Finally- make an honest assessment of your capabilities and use appropriate increments. When I use the tricep pushdown, I increment by placing a 3-4 kg barbell on the stack (if the individual plates are say 10kg each) as I can't increment effectively with such large leaps.
Relative to my physique, my triceps have come along nicely size-wise using skullcrushers and tricep pushdown in alternate cycles. However I don't use very heavy weights in the pushdown- compared to some of the monsters in the gym, but I keep the form tight, it burns like hell and the results have been solid.
