1 Didn't, i.e. did not is in past tense and does not agree with the rest of the sentence and therefore, you would need to tweak the second half for it to properly fit: Makki and Mattsun didn't have time to execute their plan, for two not-so-soon-expected guests suddenly made an outstanding entrance!
In terms of when you should use did not as opposed to didn't, I think it depends on the situation, and on your relationship with the intended audience. If you're writing a dissertation or a report to your boss, maybe use did not; if you're writing a text or email to your friend, I'm sure didn't would be just fine.
He said with didn't you don't use another past verb form. This is a good heuristic, though I don't recall anything about "double past" in school myself. But it still works. The technical way to say it is " do can take an auxillary/helping verb, but the only valid auxillary/helping verb for do is the plain or infinitive form (same as present tense)." She would hold my bicycle from the back to ...
I didn't go to (the) party I didn't went to (the) party. After the auxiliary verb DO the main verb must be in the plain form. This is the form you see in the dictionary. It does not have any tense. It is not past or present: *He doesn't goes to the gym. (ungrammatical - main verb in present tense) *He didn't saw the film. (ungrammatical - main verb in past tense) He doesn't go to the gym ...