Not sure what case law you were being pointed to, I seriously doubt it exists pertaining to the executive.
All Federal law enforcement falls under the Executive branch, Trump could gut the FBI tomorrow and put Federal investigations back into the hands of the DOJ if he really wanted to, the FBI is not a 4th branch of Government, they answer to the president.
Under the law, the President has every right to establish prosecutorial and investigative priorities. Even if Trump did order Comey to drop the case against Flynn (he of course did not) it could never be considered obstruction of justice.
Top decision makers on all levels make these kinds of judgement calls to not go after someone all the time. Elected prosecutors in all 50 States do that hundreds of times every day. And yes, sometimes a politician or other prominent citizen does sometimes call and ask a prosecutor or investigator to please go easy on someone they consider a “good guy”.