Howdy!
Wasnt long ago i had the same questions.. Since then ive only built about a dozen crank baits, but since most of them were failures, ive learned alot!
I think of a crank bait as more of a "machine" than a fishing lure.... Change one component, and the whole operation changes
The need for a thru wire depends on the material of the body.. Some guys use them for balsa baits, but there are other methods as well...... Definitely not needed with hardwood or PVC, twisted wire set in epoxy is plenty strong (ive tested mine by hanging a 30 lb dumbbell from the hook hangers)
As for where to place the ballast (weight), there are formulas and equasions here in the archives to help with that... But ultimately it comes down to testing..... When designing a new crank bait, i like to build one or two that i never plan to use.. Once i get them right, i can follow the correct steps to build the final product