The advantage of the Dick O'Kane method is that you are not using a calculated offset which could be a half degree to one degree off. You are letting the TDC calculate the correct lead angle and then are able to sight exactly where you want the torpedo to hit and press the fire button.
This is much more accurate than a calculated lead angle or an interpolated lead angle from tables. However, the lead angle isn't the true limiting factor of the technique.
The true monkey wrench in the works is the determination of target speed. And similar to the growth in significance of a lead angle error with range, the speed error also becomes proportionally more important with long range.
Because of this, and because torpedoes with stated ranges of 4500 yards tend to poop out at 3000, I limit my shots to 2000 yards, regardless of the type of shooting I'm performing.
Yeah, I've seen the movies of multiple hits from 4000 yards, and they're mighty impressive, but how many misses do you have to accumulate for one perfect video? I also don't find it difficult to make a periscope depth approach to within 2000 yards for the high percentage shot. I've never done the duck under the layer trick approaching a convoy.
Yes, it is possible to avoid a divergent spread. It is just less likely than the longitudinal spread. I try to take long distance shots at an angle that will allow misses to have an opportunity to hit other targets, and a divergent spread helps there too.
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