Can you be more specific with this statement "However, when I read the commentary it says you can use Table 3.1 fillers for multipass applications until the cap when you have to then use Table 3.3 fillers. Yet, the commentary is not mandatory, and I cannot find any such language in the body of the code that could justify this practice" there should be a letter designation or note number for this "commentary".

You could use the commentary to support the interpretation that the word "exposed" in the title of table 3.3 and paragraph 3.7.3 means that only the "exposed" weld beads are affected by the requirement.

Its done all the time to keep filler costs down.
Its not only what the code says it also what the code does not say or does or does not specifically prohibit. There are lots of sections that are misread and or misapplied costing companies thousands of dollars of additional costs. The commentary is there to let you know what the D1.1 committee was thinking when a section was written or the application it was written for. Remember the D1.1 committee writes by consensus if they can't agree it is not written in as code. (Its a "We know this works" code which does not prohibit an EOR from doing something else)

Weathering steels such as A588 & A606 or even A847 (which is not prequalified) have nickel in their chemistry allowing them (when blasted) to rust to a nice brown patina. One pass groove or fillet welds are allowed with group 2 electrodes to a limited size (3.7.3.1 & 3.7.3.2) so the % mixture of base to rod will still have some nickel in the weld to let it act as the weathering steel.
When one pass welding is done without weathering electrodes any weld repair would now require a weathering electrode. (it just became a multi pass weld)