"This" is analogous to "here", whereas "that" is analogous to "there". If you mentally substitute this/that with "this here" or "that there", one will often seem to be the obvious choice. Another way of thinking about it is if you imagine actually showing the other person the thing you are talking about, would you point to it, or hold it up to show them? If you would point to it, use "that"; if you would hold it up to show them, use "this".
For a concept or idea introduced a few sentences before, I think the key thing is whether it was introduced by you or the person you're talking to. If you introduced it, you're "holding" the idea - use "this". If they introduced it, they're "holding" it - use "that".
As for your last sentence ("Is there a rule describing proper usage of these words in cases like this?") "this" sounds much more natural and correct. I think that's because you are presenting some examples for consideration, so you can imagine that you are holding them in your hand/head and showing them to the target of your question, hence "this".
Interestingly, if I were to then refer to your examples I could use either "this" or "that". Both would sound fine, but have different implications. "This" implies that I have taken your examples/concept/idea into my own headspace to examine and consider close-up, and am still considering them. "That" implies that they are with you, rather than me; either because I'm considering and commenting on them from "over here", as your examples, or possibly that I've brought them into my own headspace, considered them, and have given them back to you prior to commenting on them. Either way, the implication is that they are your examples - I haven't taken on shared custody of them.
Against and towards can be used interchangeably in some circumstances, but I don't think this is one of them. In this case, towards would imply affection and against would imply distate or dislike.
An example of a situation where they could be used interchangeably would be "Please apply the overpayment against/towards my existing credit card balance."
As for a rule, I'm not sure there is one. Maybe if emotions are involved, don't use them interchangeably but if there are no feelings involved, then it is ok to do so
Best Answer
A plural example allows for varying interpretations. This task is incomplete and This task is not complete have the same meaning.
However, All tasks are incomplete usually means that each and every task is incomplete, while All tasks are not complete usually means that some tasks may be complete, but not all of them are