1) "courts of law" is the better expression. "law" can refer to many things: laws of physics, laws of grammar, moral laws, etc. Also, justice is usually executed in the court system. So it is best to be specific and say "courts of law".
2) No, you can't change it. The written text is saying that the "status quo" is that law is an abstract concept. And only rarely does it cease (stop) being that. Your sentence: "There are many instances when justice does not cease to be an abstract concept." - it is too complicated. "cease" means "stop", so "does not cease" means "does not stop", which is the same thing as "continues". So you are saying ""There are many instances when justice continues to be an abstract concept." = "Justice is an abstract concept." When you talk about a "rare instance", it is like the exception to a rule. So you need to emphasize that it is rare. Sorry, this is very complicated to explain.
3) Yes, it should be "muffled". It appears to be a typo.
4) ".......she ran to tell the manager who promptly rang up the fire brigade." is correct. There are two parts: 1) she ran to tell the manager; 2) the manager rang up the fire brigade. If you use "promptly", it means the second part happened immediately after the first part. "who" is used to joined the two sentences together.
she ran to tell the manager
the manager promptly rang up the fire brigade
she ran to tell the manager, who promptly rang up the fire brigade
You can use "who" to combine the sentences into one and avoid repeating the noun "manager".