Causation lies in the past/present and purpose in the future (the preposition para literally came out of por a1 and the preposition a added the destination-y connotation).
So let's take some of your examples and reduce them to minimal pairs and compare:
- ¿Por qué estudias español?
- ¿Para qué estudias español?
In the first, I want to know why, that is, what has caused you to want to learn Spanish. Perhaps you have friends that speak Spanish or you got interested from a vacation trip to Cancún. In the second, I want to know to what end (or whither, in pre-1900 English) you want to study it. Is it to get a better job? To be able to talk to your grandmother who's a Spanish-speaking monolingual?
- Trabajo para dinero.
- Trabajo por (lo del) dinero.
Here, por has two potential meanings, and its non-causitive one will generally prevail entirely (that is, it will virtually always be interpreted as in exchange for, but by adding lo del we have a better but not guaranteed chance of getting a causative interpretation sans context). But given the causative por, it means that I'm working because I'm poor, or because I've got no money, etc; that is, the money —or lack thereof— causing me to work is in the past/present and chronologically is situated before my work. When I work para dinero, I'm working in order to get money and the money is a future end-goal situated chronologically after my work.
You could almost generalize it as saying that when given a sentence in the form of [action X] por/para [thing/action Y], we are saying that with por, Y causes X to happen, and with para, X causes Y to happen.
That's why the other two sentences you have wouldn't really work with para, because there's not going to be any common rationale by which the action done causes or benefits the object of para. It has to be that studying it is done because of the love of Spanish. This is also in part because para is virtually always used in its causative sense with verbs and not with nouns, while por can be used with either (with nouns, para in such a sentence will generally mean for the benefit of)
1. Interestingly enough, the literal translation of which —for to— can be heard in some dialects of English today with the exact same meaning. "I'm off to the store for to get some milk"
