Is it משלמים את עצמם or משלמים על עצמם?
To my ear משלמים את עצמם sounds like the machines pay themselves, as opposed to paying for themselves
You're right, in my opinion "משלמים את עצמם" shouldn't be accepted.
I wouldn't use either "משלמים את עצמם" or "משלמים על עצמם" in Hebrew. Both are direct translations of English idioms and neither has become commonly used in Hebrew. The verb לשלם is not used to mean "earn" If I wanted to express the meaning of the English sentence I might go with something like:
המחשבים האלה מחזירים את העלות שלהם
I was under the impression that אלה means "these" while אלו means "those" but Duolingo seems to use them interchangeably. What's the proper use of אלה vs. אלו?
There is no difference between אלה and אלו, they are words from different phases of the language and the fashion on using them alternates from time to time. "Those" would be ההם / ההן. By the way, I would say משלמים על עצמם.
Both על and עבור are fine, there isn't a big difference in meaning, but עבור is more formal. את shouldn't be used here, it's the wrong preposition.
It is not changed. There is משלמים את עצמם still. Can the main translation above be changed? Something as These computers pay themselves.
From Oxford dictionary "(of a thing) earn or save enough money to cover the cost of its purchase."
I wouldn't say משלמים את עצמם in Hebrew. As a native speaker I couldn't understand what the sentence wanted to say at first.
I notice that the direct object marker את very often is placed at the end of the preceding word when pronounced. As I understand the את belongs to the object, but clings to to the end of the verb preceeding it in pronounciation as a rule. Am I right?
I have never noticed it. It may be in sentences like this one where the next word begins with a vowel, so the speaker intentionally pauses between the את and the next word to avoid it sounding like "etatzmam".