Learn English – Did I make any mistakes in usage of tenses

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 . . . "