The intelligence assessment that Obama ordered certainly contributed to requiring a special council investigation. If it werent for Trump firing Comey, its likely that the special council investigation could have started later or not at all. The firing of Comey over "the russia thing" is the straw the broke the camel's back
Comey was fired for many reasons...take a look at how sloppy he was with top secret information...breaking today.....and the president can not be charged with obstruction for replacing the FBI chief...it didn't stop nor interfere with the Mueller probe one bit.....
Comey's memo didnt contain any classified info.
Comey was fired May 9, 2017
Mueller special council investigation started May 17, 2017.
Since Mueller hadnt started yet, there is no way Comey's firing could interfere with that. However, the FBI was investigating Russian influence in 2016 election at the time, which is why the Comey firing can be seen as obstruction of that investigation. But all the firing did was trigger more investigation.