they had to swap the engines to change the direction of the propwash, which was causing handling difficulties - stability during gunnery?
Yes, it all had to do with the power on/power off change in pitching moment. The rotation finally settled on produced the least change, and so made for a better gun platform. The original prototype direction was the worse, with the British set up in between.
Wasnt it the brits who changed the direction of the prop's on the P-38?
The British had both engines rotating in the same direction, as they wanted commonality with the P-40. Not used in combat and most saw their life out as trainers in the US with the USAAF.