I would actually recommend the opposite of what was said above. Since the meat is so thin you may have much better results pan searing it in an extremely hot pan from slightly frozen. This way the outermost layer will start to undergo the maillard reaction long before the inside of the steak reaches a medium-rare temp and will give you a better chance of preventing it from overcooking.
This link provides an overview of the process for a much larger piece of meat then your using but the searing process is what your concerned most with since your steaks are so thin. You will likely have to play with the timing a little bit with regards to how frozen the steak needs to start out but I figure erring on the side of too frozen is best since you can then heat them in the oven to the internal temp your looking for.
http://www.thekitchn.com/for-the-perfect-steak-first-freeze-it-solid-then-cook-for-an-hour-165793
There is a tradition in southern cooking for boiling Crayfish (crawdads). Boil them in seasoned water, then tear them in half, suck out the head, and eat the tail. So this is possible for shrimp as well. I have done it and seen it done at some seafood restaurants. I don't know that the heads are ever eaten, but they do contribute to the flavor.
I have also seen people from SE Asia (Vietnam, Laos, etc.) that would eat whole shrimp, shells and all. So it is culture dependent. This is true of many foods. While we balk at shrimp heads in America, we love our cinnamon and cumin (sweets and tacos). A lot of asian people I've met, specifically Japanese, find these flavors too strong or 'dirty' tasting. Culture has a lot to do with what foods you find enjoyable, mostly through comfort and familiarity.
Here is an article on taste preferences. It points out the familiarity angle, which is pretty interesting. The more foods you're exposed to before two, the more foods you'll like. After that, you tend to dislike anything new until you 'train' yourself to like it. Except Lima Beans because they are always icky.
The Psychology of hating food and how we learn to love it
While there is a lot of flavor in the head and shell of shrimp, the texture is probably an aquired taste. I also get a bit grossed out when the vein (digestive tract) is still in the shrimp. Depending on how it's processed, this may or may not be an issue. Whether it bothers you or not, you can always process whole shrimp yourself and save the heads and shells. Boil it in a stock pot with some arromatics, and you have a quick, flavorful seafood stock in about 20-30 minutes. Add some seared scallops, clams, chunks of fish, etc.
Best Answer
There are a few things you can do to help your shrimp get a nice sear.
First, make sure they're as dry as possible before adding them to the pan. Use paper towels and pat dry. Especially if you're using shrimp which you had to thaw, they can be pretty wet, and that'll cause them to steam instead of sear.
Another thing which can cause your meat to steam instead of sear is if you overcrowd the pan. Use a larger pan or do less shrimp at once. When I cook shrimp, I can see at least 2/3 of the bottom of the pan through the shrimp. If all you can see is shrimp, it's way too much, and the water it releases will get trapped instead of quickly evaporating off.
Getting your shrimp to room temp can also help, as the colder your meat is when you put it in the pan, the more your pan will cool down when you add it. A thicker pan is another solution, as it has more capacity to hold heat. Cast iron is ideal for searing for this reason. Adding less shrimp at once will also reduce this effect.
By using the fish spatula, do you mean you're pressing down on them while they're cooking? I'd skip this, it can squeeze out liquids, so it might hurt more than help. Shrimp are pretty flat, so just toss them in the pan and then leave them alone until they need flipped.
Lastly, I might skip the butter and only use an oil which can tolerate high heat. Butter will burn at high temps, so you have to keep your pan cooler, which means less of a sear. Your butter/oil should not be smoking at any point, that means the temp is too high for that type of oil. Shrimp in butter does taste better though IMO, so you can try changing this as a last resort.