Do multiple current flows exists in a conductor? Does a slower current provide extra resistance? If you have a running marathon with lots of people the slower people are in a sense an obstacle for the faster people behind them. The slower people create extra resistance.
Is there an analogous effect in a current in a wire? Are there flowing ions or flowing charged particles that have more inertia and thus provide extra resistance to the current of electrons that hit them from behind? How heterogenous is current?
 A: There are two main contributions to electrical resistance, which are the scattering of electrons due to lattice vibrations (phonons) and scattering due to impurities.
There is also electron-electron scattering, but it is minor.  From Kittel, Introduction to Solid State Physics, 5th Ed:

It is an astonishing property of metals that conduction electrons, although crowded together on 2 angstroms apart, travel long distances between collisions with each other.  The mean free paths for electron-electron  collisions are longer than 10$^4$ angstroms at room temperature and longer than 10 cm at 1 K.  Two factors are responsible for these long mean free paths...the most powerful is the exclusion principle, and the second is the screening of the coulomb interaction between two electrons.

The exclusion principle operates from the fact that scattering electrons must conserve momentum and energy and thus whether there are actual states available for this to happen becomes important.  If there are no states for a certain collision to occur because they are already occupied, then the exclusion principle says this collision won't happen.
So the velocity distribution that exists in the electron gas that forms a current has little effect on the current itself.  (Note: the velocity distribution would be due to some electrons scattering with either a little shorter or longer mean free path from the scattering mechanisms first described.)
If you go to very low temperatures, you can begin to see effects of electron-electron collisions.  Again Kittel writes:

At liquid helium temperatures a contribution proportional to T$^2$ has been reported in the resistivity of indium and aluminum, consistent with the electron-electron scattering cross section.

Charged impurities could certainly hop from vacancy to vacancy and they would move in an opposite direction from the electrons, but again here the main effect would still be standard impurity scattering.
