Could Eris have a permanent atmosphere?
As far as I'm aware, there's no direct evidence of an atmosphere at the current time. The abstract of Sicardy et al. (2011) "A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation" gives a limit of ~1&nbsp;nanobar on the presence of methane, argon or nitrogen atmospheres during the stellar occultation observed in November 2010.

Hofgartner et al. (2019) "Ongoing resurfacing of KBO Eris by volatile transport in local, collisional, sublimation atmosphere regime" argue that at aphelion there would be a local atmosphere over the warmest region, which they state is a similar situation to the atmosphere of Io. They predict that this local atmosphere could still have significant effects on the volatile distribution on Eris:


  The model results indicate that volatile transport (VT) on Eris, even at
  its aphelion distance of nearly 100&nbsp;AU, can be significant. The nitrogen-ice
  temperatures are &lt;&nbsp;30&nbsp;K and vapor pressures are &lt;&nbsp;10&nbsp;nbar but the significant pressure gradients in the local atmosphere regime result in transport of
  nitrogen mass, that integrated over the long timescales associated with such
  a distant orbit, can be significant as compared to the column mass of the
  atmospheres of Triton and Pluto.


So depending on where you are on Eris, the atmosphere (if there is one) may or may not last for the entire orbit. It will take a long time to confirm this, though.