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Continuing on, let's check higher masses as well. For a 10 Mearth P9, orbital parameters have to be fine-tuned to even remotely match the data, while a 20 Mearth P9 appears to be fully ruled out by the current dataset. 
An example of a pretty good-looking simulation is shown below. Observed dynamically (meta)stable KBOs (large purple, gray dots) indeed look like they've been randomly drawn from the phase-space distribution of simulated observable long-period KBOs (blue points).
Of course, if this is right, then under the influence of P9’s gravity, the real objects should behave like the simulated ones. And you know what? They do.
Perhaps the most unexpected consequence of P9-forced evolution of the distant solar system is that it naturally generates highly inclined KBOs like 2015 BP519. Turns out, however, the efficiency of generating such objects is strongly dependent of Planet Nine's eccentricity.
Once excited to high inclination by P9, distant KBOs can get scattered inwards by Neptune, dramatically shrinking their orbits. Predicted P9-pollution of the Kuiper belt by high-i objects is shown in green on the plot below. Black points are the observations. You decide.
If Planet Nine is smaller, does that mean it's harder to find? Counterintuitively, it's the opposite. The smaller distance from the sun more than makes up for the diminished radius. Annoyingly, though, the aphelion is very close to (in?) the galactic plane (where stars confuse).
Assuming P9 is really there, how did it get to its strange orbit? There are 3 scenarios that have been discussed in the literature: in-situ formation, scattering out, and capture. At this point, it's still speculative, but the middle option seems most plausible.
Of course, there are many subtleties and details that I've skipped over, but it's time to wrap this up. If you're interested in learning more, I warmly invite you to check out the paper itself (it's only 92 pages long): arxiv.org/pdf/1902.10103…
I will leave you with one final thought. Suppose you had never seen the Kuiper belt, but were nevertheless wondering where in the solar system you could hide a planet... Turns out, ephemerides, stability, and definition of a planet alone give you a pretty good hint!
