Let's start by looking at the United Stats Attorneys' Manual published by the Department of Justice and providing everything you wanted to know (or in yoru case perhaps DIDN'T want to know) about the US Attorneys.
3-2.100 United States Attorneys
The United States Attorney serves as the chief law enforcement officer in each judicial district and is responsible for coordinating multiple agency investigations within that district.
There are currently 93 United States Attorneys stationed throughout the United States, Puerto Rico, Guam and the Northern Marianas. One United States Attorney is assigned to each judicial district with the exception of Guam and the Northern Marianas, where a single United States Attorney serves in both districts.
3-2.110 History
The Office of the United States Attorney was created by the Judiciary Act of 1789 which provided for the appointment "in each district of a meet person learned in the law to act as attorney for the United States . . . whose duty it shall be to prosecute in each district all delinquents for crimes and offenses, recognizable under the authority of the United States, and all civil actions in which the United States shall be concerned . . ." 1 Stat. 92. Initially, United States Attorneys were not supervised by the Attorney General (1 Op.Att'y Gen. 608) but Congress, in the Act of August 2, 1861, (Ch. 37, 12 Stat. 185) charged the Attorney General with the "general superintendence and direction duties . . ." While the precise nature of the superintendence and direction was not defined, the Department of Justice Act of June 22, 1870 (Ch. 150, 16 Stat. 164) and the Act of June 30, 1906 (Ch. 39, 35, 34 Stat. 816) clearly established the power of the Attorney General to supervise criminal and civil proceedings in any district. See 22 Op. Att'y Gen. 491; 23 Op. Att'y Gen. 507. Today, as in 1789, the United States Attorney retains, among other responsibilities, the duty to "prosecute for all offenses against the United States." See 28 U.S.C. Sec. 547(1). This duty is to be discharged under the supervision of the Attorney General. See 28 U.S.C. Sec. 519.
3-2.120 Appointment
United States Attorneys are appointed by the President with the advice and consent of the Senate for a four-year term. See 28 U.S.C. Sec. 541. Upon expiration of this term, the United States Attorney continues to perform the duties of the office until a successor is confirmed. United States Attorneys are subject to removal at the will of the President. See Parsons v. United States, 167 U.S. 314 (1897).
So you now have some specific legislation and case law which supports my assertion that the US Attorneys should support the prosecutorial directives of the POTUS (via the Attorney General). You also have specific information regarding their removal at will by POTUS. If in fact, the specific US Attorneys in question here failed to properly prosecute various Federal offenses despite being informed by the AG that they should be prosecuting them, those US Attorneys would have failed to "prosecute for all offenses against the United States, " thus their dismissal would fall under the auspices of merit.
Now if POTUS or one of his assigns had indicated that particular cases should be pursued or dropped, there might be a question of obstruction, but as long as the instructions are based on general policy (such as "prosecute illegal immigrants" then there is no obstruction. At that point the only question would be if the policy would violate existing law in which the policymaker could be liable for acting illegally under the color of law.