I don't have Valve Index, yet. I have five vr headsets (including DK1 which I think stopped working) and I was really tempted to buy Valve Index. Well I still am ;)
If you could run the game with option --drawVRControls (check run_steamVR.bat, copy it, change option to --drawVRControls). When you start the game, you will see a debug display for controls. There should be a few bars on top (for fingers), square zone for joystick and a lot of numbered buttons below. Try pressing conrols for grab, check what has changed and let me know.
Also, if you could send me a log file to contact@voidroom.com . I want to make sure the name that Index controllers use to identify is recognisable by the game.
VR : MISSING HAND POSE appears when pose is not provided by vr system. It might be because hands are out of cameras' sight. In such situation it is assumed that the hand pose is the same, relatively to the head.