Next time you have a problem screw dont use solvents, dont cut the screw off and dont hacksaw a slot into the screw. These all leave a very large margin for error and screwing up the knife like you have found out. If you ever run into this situation again simply grab a soldering iron, let it warm up. Then apply the tip of the soldering iron to teh the head of the screw for anywhere between 5 to 10 seconds and then try to back out the screw. If it still wont budge do it for 20 seconds and it will back out. The reason the screw wont back out is because it has thread lock on it which is a variation of superglue. Heating the screw will help melt the loc-tite so you can remove it.
Here are the reasons why the previous recommendations are prone to failure.
1. WD40 is a weak penetrating liquid that is completely useless against loctite. Great for rust but by spraying lubricant on a bolt with loctite you just create a more slippery surface for your tool
2. Cutting a flat notch in a screw causes a few problems. first you risk damaging other surfaces of the knife. secondly if cut too deep it will weaken the bolt and you could sheer the head by trying to twist it. And lastly a torx bit driver is designed to do exactly as the name implies with is to increase torque with less power. This is why great care is needed when using small torx bits as they easily will either strip the tool or the screw. If the bit with its amount of power wont move the bolt it is sign there are other problems that need addressing instead of modifying the screw.
3. You NEVER want to sheer the head of a bolt off to remove it. Especially on a clip. The reason is that a clip is usually so thin that after you remove the head of the bolt you will not have enough material to grip pliers or vice grips to. And even if you do you wont have enough grip to get it to move if your bit didnt do so.
If for some reason you actually strip the head of screw the best course of action is again to apply heat and to purchase a small screw extractor from the hardware store. You apply the heat first and then use the extractor to back it out. If you sheer the head the only thing left is drill out the screw and retap for a larger one.