Mixing in an untreated room = headphones.
That would not help to judge imaging, depth-of-field, reverb, ambience, localization properties and bass character of the mix. In many cases, loudspeakers are mandatory.
I found the Kali LP 6 having a pretty smeared transient response.
Could you describe more precisely what you mean by that, which instruments/sounds in the mix are mostly affected, in which frequency bands you hear this, or could link to exemplary recording pls?
Could make a wild guess that boomy bass, slow decay and room modes are contributing to this problem, but this is not precise enough to understand what should be altered.
What spaker do you think would perform the best in my unfortunate scenario?
It depends a bit on the actual frequency bands in which you are having problems and what is your listening distance. From own experience I would try something like a MEG RL921k or KSD C-100 Reference. If you listen in a nearfield scenario, a smaller model like KSD C-5 Reference would be advantageous, but you have to make a sacrifice regarding bass extension (for electronic music or anything with powerful beats, I would not recommend a compact cardioid model).
That's what a minimum phase 3 way does.
So maybe a linear phase model, closed box bass concept or cardioid would be a better choice if the OP´s description of ´Smeared transients´ originate from bass reflex?
if one gets a UMIK-1 and can turn down room modes/doesnt have the worst room ever, they can get a good response.
That is unlikely to happen if the monitors already come with inherent transient problems in the bass bands hence subjectively ´slow, boomy, delayed, overly fat bass´. EQ almost never really solves any decay or room mode issues, just makes it less annoying. For a mixing environment, particularly when judging bass beats is an aim, an EQ´d version can make things even more difficult and lead to misjudgments in the mix. I guess it is obvious why this is the case.
Vertical lobing is definitely of less importance than horizontal errors. The KH120 II is fine.
Don´t quite understand how you get to this conclusion and what you base your verdict ´definitely´ on. As the boundaries causing vertical reflections, like floor, ceiling, mixing console, desk or alike, are much closer to the loudspeaker and in the majority of cases offer a lesser degree of absorption, it is counterintuitive to think that vertical reflections are insignificant compared to horizontal ones. I would rather say the opposite is the case in a nearfield or untreated environment.
Have you used the KH120 or a similar concept for mixing in a nearfield environment, comparing them to true nearfield monitors? I have for years, in the old days when such 2-way concepts were kind of a standard, like the predecessor labelled K+H O110, as well as Genelec 1030A/1031A and Mackie HR624/824. Doing a direct A/B comparison to proper coaxial models was really an eye-opener for me, and I would never want to move back.