As an electrical engineer, I can tell you that it is perfectly fine to plunge your soldering iron into plastic, and wiggle it around to get a bigger hole. Just remember to clean the plastic off the iron soon after, so it doesn't permanently onto the iron. Also, the melted plastic will break off easily after it cools a little - after it cools all the way, it is easy to break off more than you wanted to.
It is interesting to me that you've stayed with a QWERTY format, as opposed to Dvorak, or something like that.
I've been typing for a long time (started BASIC programming in 1st grade or so), and so I never took a typing class, and having horrible typing behavior, as defined by a what a typing teacher would say, but I type quicker than most people. I've tried a couple times in my life to improve my typing behavior, and force myself for a week to type "properly", etc, but I haven't been able to get my speed up to what I can do with my random patterns (letters are typed by different hands based on what the other hand is doing).
The one question I have for your keyboard is - if you have a different layout than other people, do you have issues when using other keyboards? Do you have a mobile device - once you are switching to a soft keyboard, does it not conflict with your brain knowing where the keys are because it is completely different? I suppose the QWERTY pattern is still there. (I had terrible trouble with Dvorak, and stopped using it due to using other people's computers).