Your first example:
But it would be a surprise if the presidential election scheduled to take place in Venezuela is allowed to threaten the position of the country’s dictator, Nicolás Maduro.
You are right to assume this is a type 3 conditional, according to the classifications in your book. If the Economist writers were following the strict rules that your book proposes, then the proper way to write this sentence would be:
But it would be a surprise if the presidential election scheduled to take place in Venezuela were allowed to threaten the position of the country’s dictator, Nicolás Maduro.
I would have written it that way, although as your text pointed out, many English speakers would have written was. Still, I don't think most speakers would combine "would" with "is." I'd say it's a grammatical error. Either "will" with "is" (which would make it a type 2) or "would" with "were" (which would make it a type 3) is much more correct here.
Your second example:
Finding a viable way to mine outer space’s plentiful supplies of platinum, for example, would surely lead to a meteoric descent in the price of the metal.
This is another form of the type 3 conditional, although your book doesn't seem to cover it. The first part isn't worded using an "if" statement, but a gerund phrase like this, followed by "would" is another proper way to describe something imaginary. It could be reworded like this without changing the meaning, at all:
If we found a viable way to mine outer space’s plentiful supplies of platinum, for example, it would surely lead to a meteoric descent in the price of the metal.
Note that you can also use infinitive constructions this way: "To find a viable way . . . would surely lead . . . "
"Jack read a book yesterday" is correct, but not the most natural sentence for this situation.
Simple past describes an action that is finished. It does not mean the action is complete, as in, that Jack read the whole book. It means Jack started reading and stopped reading. Anything that happened in the past and is not still happening now can be described with simple past.
So, in your example, the simple past describes the event of Jack reading the book.
So, what's not natural about the simple past version? In English, when we say that someone "read a book", without any other context, we understand it to mean they read the whole book. It's not a requirement of the grammar, just a convention. If we don't want to suggest that Jack finished the book, then we have several choices:
Jack spent some time reading a book.
Jack read a book for an hour.
Jack read a book until he fell asleep.
Jack did some reading.
And so on.
In English, we tell stories in the simple past. Clauses in past continuous describe things that are not the main story, but background information or context for the story itself.
So in a sentence like "Jack was reading a book when his phone rang" "was reading" always means that reading the book is not the story I'm telling, just a side detail, so if the story you want to tell is Jack reading a book, past continuous is never correct.
As for how this sentence can both mean that Jack read a whole book and Jack didn't necessarily read a whole book, it depends on where you're looking for meaning. The grammar itself just means Jack read something. But if you say "Jack read a book", people will guess that it means a whole book.
It's just convention that things like "I read a book", "I watched a movie", or "I mowed the lawn" usually mean the whole book, the whole movie, and the whole lawn. But it's natural to say, "I mowed the lawn, but didn't finish the part behind the garage". This shows that it doesn't necessarily mean to completion.
If the action is something that can be completed, more often than not, that's what people will infer, but not always. "I watched my favourite TV series last night" does not imply that you watched the whole thing.
And there's no rule about how long something takes, because, "I wrote a TV series last night" does suggest that you wrote the whole thing, even though that clearly takes longer than watching a whole series.
Best Answer
Open conditionals are used when the speaker thinks it possible that the "if" clause is true. You might use a past conditional when giving an instruction.
Or simple conditionals like
Compare that to the "second conditional":
Which presents an unreal situation (you didn't start reading) but is speaking about a non-past time.
The "first, second, third" pattern of conditionals is a simplification, to help learners understand that past tense in conditional expressions doesn't always refer to past time. But not every conditional expression fits this pattern.
Your example is, perhaps, not very common. But it is correct. It would be better to use a present perfect, as there is a strong connection to the present state:
I've also put a modal in the advice, for extra clarity.