Note to apchelopech, quit while you're even. Carefully read some of the FAQ's and old threads. Like said above, I think you are actually thinking of number agreement.
Here is the opening paragraph from "The Rope," a story of mine, set in the past tense, that appeared in Realms of Fantasy and a best-of-the-year anthology:
Past, present, and future! It can be done. Your teachers, apchelopech, were mistaken.
No, it's not a rule to keep the same tense throughout a sentence.
I'm watching TV today and saw a movie yesterday.
Let's fix the toaster that broke yesterday.
He tried to keep his eyes on the road.
I can't think of a single grammatical construction that requires tenses to match. Maybe you're thinking of number? The number of a subject and verb must match.
Note to apchelopech, quit while you're even. Carefully read some of the FAQ's and old threads. Like said above, I think you are actually thinking of number agreement.
Iánheh leapt up. The rope stood there still, but the boy was gone from the end of it. Iánheh said to herself, "Where has he fallen to?" but the ululation was of overwhelming joy.
You say your story is 'set in the past tense'. But it seems this is not the case, for here is a slightly later extract -
" Rope-charming is a profession not unknown in this part of the world, but it has never been a common one. Most rope-charmers will use cordage, thin rope or often ordinary cotton twine, not longer than five or six yards. (The precise length of the little khedeev's rope had not been determined, but its diameter was in excess of two inches.) Some rope-charmers use drums, and others reed pipes, to "draw up" the rope. The little khedeev used a side-blown ivory horn."
This is manifestly written in the present tense (the portion in parentheses excepted).
The past tense would read -
"Rope-charming was a profession not unknown in that part of the world but it had never been a common one. Most rope-charmers used/would use cordage ..."
And if you choose to use, as per your first extract, 'as will be seen', you are again demonstrably in the present tense. Were you to be writing in the past tense, you would write 'as would be seen'.
I need to be more precise - I was not purporting to state a rule for all sentences (needless to say) but rather for the particular sentence on which I was commenting. And that sentence contains a subordinate clause with a verb, which follows a primary clause with a verb. And what I learned - not in the USA to be sure - was (and remains) that the grammatical rule required the tense of the subordinate verb to conform to the tense of the primary verb.
None of the three examples you give are in the same category.