A key to these tangs is being dead soft. A key to khukuri use is technique. It takes a while to get, but works so well with the design. It involves a loose grip where the shoulder starts the chop, the elbow then continues it, and the final blow is sped up by a flick of the wrist. If done right, the hand is not putting any force on the handle or blade at contact with wood. The user just builds the momentum and the blade's speed/mass does the dirty work. It doesn't stress anything except maybe the edge as it's essentially thrown into the cutting medium with the user only controlling direction at the time of contact. I hope I explained that technique right. I'm sure someone else can do a better job, but the technique works very well and doesn't tire you out. We've all tried to force the khuk into wood, like what you're describing as putting incredible force on the handle near the bolster. With different technique, that's not an issue.
I'm not questioning your technique or skill at all, just pointing out that many here use the flick technique and it negates a lot of the handle stressing force you mentioned above.