capped nuts wouldn't make any difference - you would still have insufficient thread engagement because the studs are too short.
they shouldn't be tighter than a tight thing! 81lbft is the correct torque, which aint that tight.
the wheels are located by the action of the mating tapers on the inside of the nut and the dish on the wheel. the strength of the joint is dependent on the surface area of the mating threads - more surface (mating) area = more friction = more strength.
I wouldn't be happy belting down the motorway without full thread engagement on me wheel nuts!
PS - I once had both front wheels come off me mini simultaneously at 80mph, the front end dug in and it flipped end-over-end several times. fortunately, it was an oval track racer and I had a full harness, roll cage and crash helmet.