Too many exclamation points! They make the writing look weird! They also make the characters seem overly excitable!
On the other hand, an exclamation point would at least not be incorrect after true, whereas the question mark definitely is. Do not use a question mark unless it’s actually a question. Expressing disbelief is not the same as asking a question.
Sometimes it is appropriate to use both a dash and either a question mark or an exclamation point, but only when the sentence is cut off abruptly, and then you’d use the dash before the other punctuation: “What the –? This is the most ridiculous –!”
I corrected everything in the sample except the overuse of exclamation points:
“Come quickly! The borders to the other world have opened!”
“Surely this cannot be true! They have been sealed for centuries!”
(When — please note that I do not say if — there is occasion for a similar conversation in my own fiction, you may rest assured that the characters involved will not be so… exclaimy.)