After reading your comments and mine, an objective observer probably would conclude that the White was "between the lines" from mid-summer 1862 to mid-summer 1863, though I would give the edge to the U.S. Navy because of its ability to push up the White whenever it wanted to.
I can't provide any additional information about the Ponchartrain, but let me offer a suggestion about Thomas Selfridge's state of mind. Selfridge was young and ambitious (first in his class, USNA '54). In December 1862 his meteoric rise up the promotion ladder came to a temporary halt when he lost the Cairo to a mine (two mines, actually) in the Yazoo. The Cairo was the first of the city-class ironclads to be lost and Porter was upset about it. Selfridge was anxious to redeem himself, and I suspect that he thought a dash past Fort Pleasant to sink the Ponchartrain was just the sort of bold action that would offset the loss of the Cairo, impress Porter, and get his career back on track.
As for Porter's intense interest in the Ponchartrain, keep in mind the sinking of the ironclad Indianola in the spring of 1863 by the wooden gunboats William Webb and Queen of the West, which operated out of the Red River in Louisiana. (Quite a spectacular feat, by the way!) The Ponchartrain was very similar to those two Confederate vessels, and its geographical situation was almost identical. All of this likely made Porter more interested in the Ponchartrain than might normally have been the case. He was concerned that the Ponchartrain (a la Webb and Queen)would dash out of the Arkansas, strike a blow, and then scoot back up the river.
In other words, perhaps there was a psychological dimension to the thoughts and actions of Selfridge and Porter.
By the way, I reviewed the Arkansas section of the Vicksburg study for the NPS. Thought it was quite good, though I found a couple of glaring errors. Neither concerns our current topic of conversation, however.