I agree that the White River was the western edge of the "No Mans Land" in Arkansas in from May 1862 to August 1863. I also agree with your assessment of Selfridge's personal interests. Certainly his assignment as commander of the USS Conastoga was a let down after commanding the USS Cairo. I believe that he for a short time, maybe a couple of weeks, had commanded the USS Monitor. I may be wrong on that thou.
But to get past that, Selfridges efforts at self importance was not uncommon among commanders of both side to try to imbellish their roles in the war. However, Selfridge was assigned a duty to patrol the mouth of the Arkansas and White Rivers, which in and of itself was a strange assignment from a military view point. There were several suggestions to better handle the situation of control of these rivers ranging from Attacking up the Arkansas River to Capture Little Rock, to moving the entire Union garrison from Helena to Napolean, Ar. to erecting a boom chain across the Arkansas to prevent just such an incident as the Ponchartrain from making a "raid" into the Mississippi.
Moving the Union Garrison from Helena to Napolean made the most logic, because it would have offered better control of the lower Arkansas and White Rivers for Union purposes, and would have been more centrally located between Memphis and Vickburg for coverage of the Mississippi River steamboat traffic suppling Grants Army and Porter's Fleet. From all the incidents of Confederates capturing Union boats and steamers on the Mississippi even from Land bases with units such as Cavalry and Artillery batterys, one has to reasonablely wonder how effective these union gunboat patrols were in the first place. Helena was a poor choice of location for a military base of operation in that any operations from there were quite predictable. Helena's only important in the war was only because the Union Army was there thanks to Sam Curtis. Little Rock would have been far more easily taken from Napolean that from Helena. But that just my opinion and that's another discussion.
IF Selfridges sole purpose in his reports was to build his personal esteam, I do not feel that he would have been the commander of a Rowboat for very long under David Porter's command. Gunboats of any kind (City Class Ironclads or not) were too important an element of the Union Fleet to give to just anybody to command. Politics not withstanding, they just didn't have enough of them to assign to duty that wasn't important. Even the captured Confederate ones like the CSS/USS General Bragg. So why tie up 2 gunboats, the Bragg and the Conastoga, steaming back and forth between the White and Arkansas river mouths for months on end?
This is not good use of military resources without a GOOD reason. Do you see what I'm talking about? This was a reasoned decision by the highest of the Union Navy command structure and maybe even higher to handle this problem in this manner. Selfridge motives not withstanding and his personal reasons for all of his reports being unimportant. He was still there for some higher military function than simply to put Thomas Selfridge on "ICE". Why?
It is certain that the mighty Union Gunboat Fleet could have sailed up the Arkansas to little Rock at anytime they wanted to and eliminate any threats whatsoever. Why did they choose to simply put "the Cork in the Mouth of the Bottle" method so to speak? I have a couple of ideas on this but some of this hinges on the answer to the Question of, How much of an actual precieved threat from a "CSS Webb style attack" was the Ponchartrain to the Union Navy high command? It is clear that the Union navy chose not to Close the Arkansas River completely, But on the other hand they were concerned about something other than a Confederate Flatboat coming out of the Arkansas River.