The Battle of the Margus AD 285, depicted on the Arch of Constantine

To me the absence of other emperors is less of an issue, as the vast majority of surviving friezes on the Arch of Galerius also lack co-emperors (and anyone adapting Diocletianic friezes for a Constantinian monument erected in 315 would naturally avoid any equivalents to the panel of the emperors enthroned in Thessalonica). But the sheer correspondence between the Sol-Victory combination and Constantinian iconography I think has won me over (although their argument that Diocletian did not much honour Sol is undermined by their failure to consider coin types that represent Sol without specifically naming him).

Hmm iirc 40 per cent of statue bases to Tetrarchs are to only one ruler. Tetrarchic media varies between individual and collective credit (this why each vicennalia column depicts an individual Tetrarch, for example). It just wouldn't be very surprising to me that there would be a monument with enough friezes depicting a Tetrarch on their own to be spoliated in this way. Other parts of the arch could honour the others. The vicennalia was shared by the two Augusti, but it was Diocletian's accession that was actually being commemorated, a good fit for the Margus. And no other Tetrarch has any business appearing in a series of Margus scenes, like Galerius in the east. But yes, the lack of testimony for such a monument is a problem.

That said, I thought it was interesting how they reviewed the evidence for the Margus battle and manage to cast doubt on whether it was fought at the river.

The vicennalia monument is very incomplete. It points to a show of collegiality (which is also true of the Arch of Galerius, which is nevertheless more heavily packed with individualistic scenes despite being a tetrapylon), and it demonstrates senatorial concerns (it's a senatorial monument; a hypothetical Diocletianic arch is not necessarily a senatorial monument, regardless of its hypothetical final destination on the senatorial Arch of Constantine). The surviving Tetrarchic arch (albeit incomplete) that we have is the Arch of Galerius, a celebration of victory like a hypothetical celebration of the Margus, and so I'm not as inclined to put superior weight on the vicennalia monument. But regardless of who dedicates the monument or statue or speech, individuality remains in the picture. This isn't really a matter of individual monuments needing to exist, but monuments that include representations of individual achievement, which was very much in the Tetrarchic playbook (something that is increasingly being given attention on scholarship). For example, the surviving Panegyrici Latini focus more on Maximian or Constantius than collegiality. If the event being presented is unique to one of the Tetrarchs, one would expect a considerable amount of imagery focused on that ruler, as in the case of the panegyrics, or the Arras medallion, or the Arch of Galerius, where only three friezes acknowledge other emperors among the numerous friezes present. Plenty of individual scenes exist in the Nicomedia friezes. The Rome context is important, but available material is fragmentary (thus e.g. the Arcus Novus - although this was indeed dedicated to Diocletian, like the Baths). Collectivity wasn't ignored, but I think it's sometimes overemphasized. In other words, yeah it's a hypothesis that Diocletian was honoured with friezes honouring him as an individual victor over Carinus, but I'd also say that an unwavering predominance of collegiality also relies on incomplete evidence. Monuments would honour collectivity, but many would probably include individual-focused scenes as well. I must say that I also disagreed with the confidence with which they dismiss the Margus as an event of importance to Diocletian. It appears often in sources, even though Diocletian didn't have the level of influence on the historical record that the longer lasting Constantinian Dynasty had, and crucially we have no surviving panegyric to the emperor to whom it's directly relevant. While we might question whether Margus was still a hot topic in 303, I suspect it was indeed a legitimating event, considering how much Carinus was painted as a tyrant. That's one of the most irritating things about the Tetrarchic period, the lack of panegyrics to Diocletian.

I get you, and I agree regarding the Oratio and Liberalitas that it's more challenging to associate those scenes with Diocletian alone (unless those scenes relate to the visit to Rome in 285 just after Carinus' defeat, although if that is so than realism wasn't the concern, considering the presence in the frieze of the vicennalia monument - the Chronica Urbis Romae does not record a congiarium at that time, but it does not seem to be well informed on Diocletian's early reign (perhaps due to the fire?), and Burgess, the expert on Late Roman chronicles, thinks that the Chronica is a problematic source). Personally I would still take private dedications into account, as monuments, statues, coins, souvenirs, etc display numerous commonalities and patterns, especially when evidence is fragmentary, since they build a broad picture of how people represented the Tetrarchs and how the Tetrarchs wanted to be represented, which can be analysed an sich since those two dynamics of representation fed into one another (thus why private dedications still pick up on wider practices, like the formula to 'their (plural) maiestas', appearances of the divine signa, and Galeria Valeria being mater castrorum. But certainly context, agency and media type is important, and one could cite many notable differences in practice across media, as in the case of the signa. All this to say that, I get where you're coming from, but those are the reasons why their collegiality argument didn't land as decisively with me personally (and I think this relates naturally to the fact that we approach visual media with different interests, with you focusing much more on context, thus the emphasis you place on things not lining up well within said context). But I still am essentially convinced by their overall article.