Learn English – Which one is more idiomatic – “Occurs to me” or “Occurred to me”

It's the second meaning in use here, that is:

Pass over (a stage or obstacle)

In this case, the "stage or obstacle" is the "pain" involved in the learning curve. The sentence is essentially saying:

It was a lot of work the past year, but, if you learn from what we've learned, it might not necessarily take you a full year to accomplish the same thing.

where the authors have elected to shorten "hassle of learning" with the single word "pain."

The phrase "leapfrog the pain" isn't necessarily idiomatic, but, in this case, pain isn't much different from barriers (at least not syntactically).

That said, the phrase "blood, sweat and tears" is very idiomatic, and it means "a tremendous amount of hard work and effort." It's important in this sentence, because it helps the reader understand what pain is referring to.

You'll get an experienced voice on how to gracefully exit from a thread -- and find out when to use the lead-between-the-eyes SureStop class instead.

The meaning is "deadly" or "lethal" (adjective). Deadly to the thread that you want to quickly and surely halt.

You'll get an experienced voice on how to gracefully exit from a thread -- and find out when to use the deadly SureStop class instead.

The SureStop class apparently can be used to forcefully and rapidly shut down a thread - like putting a bullet (= lead) between someone's eyes.

Bullets are made of lead. The word "bullet" has been substituted with "lead" to make the sentence more expressive.

We can also say that this method is "deadly efficient", like last-resort methods often are. A method with a great "stopping power".

Hereunder goes a wholesale crib of a great comment by talrnu:

Military training specifically teaches shooters to aim at the space between the eyes in order to hit a particular part of the brain which instantly shuts down all mechanical control, e.g. so a hostage-taker doesn't have a chance to even accidentally pull the trigger on the gun they're holding to a hostage's head. So, shooting between the eyes is the most effective way to solve a conflict with a hostile gunman, and SureStop is parallelized with this idea to emphasize it's the most effective way to solve this particular programming problem.