Shoulders in the planche

I've recently acheived a multiple second tuck planche (though I'm still working my way up to a 60 second adv frogstand), and something has been troubling me. What exactly should the shoulders be doing in this hold? Intuitively, they should be pushing back and/or down, but when I do it they shrug forward.

Also, what are some good stretches to help forearm splints?

The shoulders should be pushed OUT (protracted) and down (so you get maximal lats/chest tension).

For forearm splints you want to lay off the exercises you were doing to let them heal and then strengthen the forearms. Rice bucket (youtube this) works pretty well. That's how I got rid of mine from too much pommel horse.