According to the traditional rule, you don't use a comma between parts of a sentence that could not stand alone, parts that take some elliptical words from the rest of the sentence. In both of your examples, the part after the comma is elliptical. However, those who follow that rule are usually not against introducing some extra comma's to aid the reader, or to indicate an afterthought or some other bit that is more loosely connected to the rest of the sentence. So I would say the comma's in your examples are optional.
However, it doesn't work the other way around: if the and... part is not elliptical, then you must write a comma. That is not optional, if you follow this rule.
Not also that your last example is ambiguous with the comma. Which does it mean?
The best mindset is to assume [that] she's attracted to you, and [to assume that] she does not play games.
The best mindset is to assume she's attracted to you. Oh, and she does not play games.
For that reason, I would avoid the comma in that sentence if it isn't strictly needed (which I think it is not: you most probably intended the elliptical construction as in (1). So I would write it thus:
The best mindset is to assume she's attracted to you and does not play games.
Or thus:
The best mindset is to assume that she's attracted to you and that she does not play games.
