I assumed you were European by your comment about the imperial system and the possible confusion on terms regarding drill size. That’s why I used pictures to avoid any confusion as not all terms translate that well.
The reverse thread for the left pedal is not very common outside of bike cranks. 9/16-20 is common enough, just make sure you have a matching tap for cutting the left hand thread on the left cranks.
I did make sure 
*I’m Scandinavian.
I took a 12.9 mm drill for the core hole. That worked out perfectly as to be expected.
The thread we are talking about here is the British Standard Cycle thread 9/16 inch 20 TPI. It’s core diameter is specified as 0.5093 inch (sic!). That’s 12.93622 mm, so let’s say 12.9 mm.
In regard to the mentioned (imperial) drills: 1/2 inch is 12.7 mm. So that’s actually a bit too small. And 33/64 inch is 13.1 mm. That’s a bit too large.
Well, you can find a number by bicyclist reporting success with a 13.0 mm drill. A 13.0 mm drill is certainly easier to obtain than a 12.9 mm one.
Nothing more to be added to @saskatchewanian and @JimT 's super graphic explanation. Nice method to keep the tap wrench straight! 
Those VCX 3 hole cranks are just regular VCX cranks with more holes drilled; multihole KH Spirit cranks, same thing; my multihole KH Moment and QX cranks, same thing. Nobody is making a special version of their crank for drilling.
So drilling your own isn’t any extra risk vs buying predrilled. Any multihole crank is going to be weaker than the single hole version, though.
But the venture cranks slamdance was considering adding a second hole into were never available as dual hole. (And definetely not a great candidate for adding one either, look at their shape, not exactly a lot of material there).
But as you said, “highly engineered” isn’t exactly the correct word for it either, as shown by the fact that (most, mad4one agile cranks are the exception) multi hole cranks are the exact shape of single hole cranks with more holes added.
So drilling holes yourself vs. predrilled ones on cranks that were already available as multihole cranks shouldn’t make much of a strength difference, as long as you stay within reasonable limits when it comes to amount and placement of holes. (putting them extremely close together won’t be great, or on KH Spirits going much lower then 110mm probably isn’t sensible either. 4 or 5 holes per crank is probably pushing your luck too, on most cranks…)
Apropos of “highly engineered”, I’ve also noticed that almost all of my shorter cranks are just longer cranks, cut off. I imagine it’s too expensive to have different forgings for all the lengths we unicyclists demand. The only exception in my collection is my 127mm Sun Isis cranks (which are, ironically, probably my cheapest cranks).
So the shorter cranks are over-engineered, since the longer ones have to be stronger to support the forces from the rider’s weight, at the end of a longer lever.
These Venture 2 cranks show what I mean - the longer ones have an eye of extra material around the pedal hole (which is good). The shorter ones don’t, because they are just cut off versions of the longer ones.
