I'm not sure how helpful this will be, but on AR's trigger slap is usually caused by the back tab of the hammer smashing down on top of the disconnector/trigger, usually from an oversized gas port resulting in excessive bolt speed. Obviously, a gas port problem isn't the case with your rifle, and if it's running, I wouldn't mess with the bolt speed or anything else. There might be a simple fix for your problem though.
Check the top rear of your disconnector and trigger for an impact mark or peening. You may be able to cut away some of the back of your hammer to provide some more clearance; this may be simpler than installing a limiting pin but you might need more spring to compensate for a lighter hammer. The other way to reduce the hammer impact on the trigger is to radius or bevel the rear of the bolt where it cocks the hammer to slow the cocking process slightly.
It's hard to see down there, but if you can check under the hammer when it's being held down by the bottom of the bolt, it should only go down far enough to engage the disconnector, and not much further. This is one of those three-hand deals, where you need to hold the trigger back while simultanously holding back the bolt and checking the hammer/disco engagement with your third hand...
Watch out for your fingers!
If the hammer is being pushed down too far it can strike the back of the trigger even when operated slowly and this will really sting your trigger finger when you shoot. It's easy to feel the hammer drag in an AR but it would be pretty difficult to feel it in an SA42. If it's down too far you might take some more off the bottom of your bolt to allow the hammer to ride higher.
Hope this helps.
Looks real good.I laughed at the remark about the string,that was how I first fired mine! 
---bil
LOL!!!