These little buggers can test your skill and patience for sure. Everyone on this forum has been there.
The frozen drive sprocket cover screws can be fairly common from what I've read here. If I recall they are quite roomy slots and need a bigger bit. Anyway the key is to really match the size of the driver used. You might have to lean into it. Possible tapping with a hammer while you turn.
Also, just in case you haven't done it, take out the plug so you can turn the sprocket without having to push in the clutch lever at the same time.
you'll get there