I also play GK and disagree. With amatuers I will agree you can look out for 'signs', usually as simple as the arc of their run but none of tht works against better people, let alone top professionals. Basic stats will show that pro goal keepers do not go the right way 99% of the time as you suggest so why you think at the top level goal keepers would bother looking out for 'signs' I don't know; and every single pro goalie I have ever heard give tips on pens has said 'pick a side a stick with it'. If a pro keeper does look out for anything, it will be if the taker is nervous and nowadays they'll research pre-game, studying a pen taker's habbits rather than waiting to see "which direction their hips are facing" during the actual pen.
Watch this, which illustrates my point of randomness quite well...
I paused it a split second before each pen and guessed the direction based on their run up...
1) Tevez - Skips round to give a wide angled run, indicates a shot to keeper's left. Result, he does shoot that way but Cech goes the wrong way (so thereby he must disagree with your theory as he went the opposite way to what the 'signs' suggested)
2) Ballack - Straightish run and short suggests to me hitting it to the keeper's right. Result, he kicks it the other way, with VDS going the same way, showing that this time the 'signs' were wrong/unreliable.
3) Carrick - Similar to Ballack but the wider angle and longer run suggested to me a strike to the keeper's left. Result, hits it to the keeper's right and sends Cech the wrong way so this time Cech followed the 'signs' rule but got it wrong thus disproving it's effectiveness.
4) Balleti - Wide, longish run and open body heavily suggest a strike to the keeper's left. Result, he does go left but top keeper ignores all the signs again showing you at that level relying on 'signs' isn't used.
5) Ronaldo - All the fancy tricks left me stumped I'll be honest but he misses which only reenforces my argument that trying tricks when taking pens isn't neccessary.
6) Lampard - Run and open body suggested strike to keeper's left. Result, did go left and VDS went that way too which is good for you. Although even though VDS did read the 'signs' in this case it still didn't help him.
7) Hargreaves - Run suggests to the keeper's left, body stance suggests right. Result, goes right, keeper goes right way but smashed into top corner.
8) A.Cole - wide run up and stance suggests a shot to the keeper's right. Result, it goes left and so does the keeper which is bad news for you
9) Nani - Short run and stance suggested to me a shot to the keeper's right. Result, it goes that way and so does Cech so one back for your theory.
10) Terry, well stance suggested it would go 5 miles wide so he did very well to hit the post!
And on your last point, I actually hold the opposite opinion. If in doubt, I would dive to my left to a right footed player as it is easier for a right footed player to side foot it to his right/the keeper's left than it is to either lace it or sidefoot drag it into the other corner.
Keepers are sent the wrong way all the time by bog standard run ups/strikes and I maintain my point that taking a pen in the manner the young Spanish guy did in the vid would make no difference to the probability of which way the keeper dives.