To be fair, the other possibility is that those Arotes melleus are in reality the redder variety of Spilopteron vicinum (S. v. melanderi). So the first step is a check through the literature.

The two genera separate at couplet 4 of the Townes' key(1) (p. 546). This boils down to differences in the hind tarsal claws and positioning of the intercubital vein. I'm not seeing that area of venation that clearly, though, and the difference is still fairly minute, all things considered. It's great for a specimen in-hand but not so great for photos. I think the last photo shows the intercubital vein too far beyond the 2nd recurrent vein to be Spilopteron.

Looking at the species, S. vicinum (p. 573, 575-576) should have the ovipositor sheath ~0.93x forewing length, which is exactly the same as reported in A. melleus (p. 567-568). There are many points that are noted under one and not the other, though it does look like the fore legs, mid legs, and hind tarsi should be more yellowish in both forms of S. vicinum, contrasting the fulvous hind femora. In A. melleus has primarily fulvous legs with only some straw-colored markings rather than strong yellow. So as none of the fore or mid legs, in particular, are actually yellow, that may point to both records being A. melleus instead. I also notice that the metapleurum of S. v. melanderi should still be mostly (80%) black. This area isn't directly mentioned in the description of female A. melleus, so that presumably defaults to the fulvous ground color, as would also be the case here.

Specimens formerly assigned to melanderi were only found about as far east as Minnesota, so that would place both records that were under Atypical S. vicinum fairly well outside the understood range of that form.

BOLD does include specimens for both A. melleus and Spilopteron vicinum (some particularly reddish individuals might represent the former melanderi).

Based on his comments on one of the linked images, it seems clear he was guessing and really was not that happy about it. Thanks for the tip. I'll go with it for now.