In the first example, you're describing your hypothetical in the present simple: "...when people read your work, they get a different perspective than the one intended*" (I have to assume "read" was in the present simple that would be correct). Also, "*It's your fault..." is in the present simple. So, "... they don't get your point" with present simple is correct.
With the second hypothetical, the answer is not as clear-cut. You switch from using real* grammar to introduce the story, "What if you buy a new car, but it breaks down while using it?" to unreal* grammar in the question, "Would you call ... or would you ask for a refund?"
As the reality of the grammar switches, it's not clear which you should use.
If you use unreal grammar in the first part, like, "What if you got a new car, but it broke down while using it?" Then, you can continue using unreal grammar: "...for the car you had bought, or...".
This is correct because the unreal form of both simple past and present perfect is past perfect.
If you tell your hypothetical story all in real grammar, then you can use simple past or present perfect: "Do you call the insurance company for the car you bought/have bought, or do you ask for a refund?"
Here, both tenses are correct, with slight nuance differences: the simple past simply describes the car as one that you bought; the present perfect focuses on the present fact that you now have a defective car because of a past action.
*I use the terms "real grammar" and "unreal grammar" because those are the terms usually used in ESL classrooms. In linguistics, they're called "realis mood" and "irrealis mood".