I like Skarner, a lot. But despite that, I don't understand him entirely to be honest. He has an amazingly short range, no proper initiate skill, and his CC is his ultimate. To even get his slow off you have to hit with it twice before it works. How can he jungle effectively without any kind of ability to properly gank?
First of all, the simplest part of your post to deal with is his Crystalline Slash. You're correct that you need to hit it twice in order to generate the slow debuff. You do NOT have to hit it twice on the enemy champion. As you run towards your gank target, hit Crystalline Slash once when it range of enemy creeps. This way, when you Crystalline Slash the enemy, you are immediately slowing.
Second, Skarner is all about the ability to quickly get on your target, since all of his abilities require being in so close. Start off by using the increased movement speed quints and putting 4 points in Swiftness. With these, plus boots 1, you will be at 405 move speed. When your Crystalline Exoskeleton is active, you'll jump all the way up to 452 move speed.
Finally, when ganking, pay attention to where you are coming from. Often you want to be moving from behind the enemy in lane. If you come out in front of them, or from the wrong side if you're ganking middle, then they can retreat in a straight line making their escape significantly simpler.
If you want to watch a pro play Skarner and learn from them, look up Diamondprox. He's the jungler on Moscow 5, an elite LOL competitive team.
A good Skarner is a terror to behold. His movespeed steroid, slow, and boosted armor (as well as his pull when he hits six) make for some ridiculous ganks even if you don't coordinate with your teammates. He's a top-tier jungler for a reason; his ganks kick ass! I don't mean to be presumptuous, but he does have a rather high skillcap and if you don't build him properly/take the correct skill order/use his skills at just the right time...he's going to fall short of your expectations. :P
I'd recommend this guide, my friend's Skarner game has improved a great deal since he started following it. The most important tip in there is to remember to autoattack as much as possible to keep your cooldowns lower, making it much easier to land your Q for a second time and get the slow off.
Even at low levels, his Q combined with a blue ( and/or red buff ) is basically a permanent slow. If you see an overextended enemy, W into lane and use your initial Q on a minion to gain the slow proc on it.
Post-6, all you need is either flash or a Shurelya's. For extra slows, a Rylai's is always nice, of course. Combined with the slow from his Q, enemies are made almost immobile.