Is it appropriate to post answers/comments about the actual application of laws?
Yes. That fits the categories of "statutes or court decisions" and "historical legal applications", both of which are listed as on topic.
As you correctly point out, oftentimes there are significant discrepancies between the theory (i.e., what shall happen, according to the law) and what the judiciary does in court. That type of posts contributes to dismantle the inaccurate beliefs one would hold from looking just at the laws that were issued properly.
Would that kind of thing be off topic as opinion based, or on topic if listed as such (De facto there seems to be times where political concerns...or something like that)?
It depends on the sources, and arguably the rationale, on which the post is premised. Mere admission of being opinion-based does not render a remark on topic, although it signals that the author does not intend to disguise his personal opinion as fact or as source.
Subjectivity and speculation are discouraged. A priori, posts that factor a party's public or sectorial role may seem speculative. But if a post provides verifiable sources that some judge at a court hearing blatantly lectures something like
The State loves [...] oil companies and it loves anybody who's
powerful. Okay? And it doesn't say that anywhere in our Constitution.
But if you're not in those groups, then you just kind of have to try
and stay away -- stay out of the way
then it would be dishonest and unreasonable to treat that post as subjective, speculative, or otherwise off-topic.