Learn English – They used to be / were good friends

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 . . . "

I recommend using the version with commas.

Typically semicolons are used to set off independent clauses (a.k.a. main clauses) that have some relationship with another such that a full stop isn't desired. Another use of semicolons, to separate items in a complex list instead of commas (because the list includes items that themselves use commas), also has support, but that's not the case in your example. Here's a brief article from Lexico in support of the foregoing.

So, the semicolon version looks very wrong to me - it looks as if you're trying to separate two independent clauses, but the second clause is not independent. The dash is relatively versatile and your use of it in your final version looks ok, but I think the comma version is best.