There are a number of examples of varying degrees of complexity from various games.What will this system be like so that it is malleable enough to apply various wildly differnt situations, yet still lends the specificity and structure you desire, and does so without stillfing the roleplay?
The GM, of course.And who decided when the situation warrants the use of this system,
I am not sure what you mean. There are always going to be problematic examples, but they will be outliers. Haven't you ever had to adjudicate a weird fight, like in zero-G in a game without rules for it, or whatever?(and why it it can never used when I come up with a problematic example)
Because the rules give strong guidelines when to use them and not. Have you ever played Savage Worlds? It has both a highly detailed grid based combat system, as well as a quick combat system. It has a detailed chase mechanic, as well as vehicle combat rules. The guidelines in the book suggest when to employ them.and how such decision to use or not to use it is not a similar GM fiat we tried to get away from?
Honestly at this point I am having trouble understanding what you are arguing against by way of interrogating a hypothetical system.