Actually, the way I see it is that all of Eevee's Evolutions are Deviations from it's natural course.
In a way, Umbreon and Espeon are actually Eevee's only "true" evolutions, without being "forced" to evolve by Stones.
If Eevee's Genetics are considered unstable then why shouldn't it be reasonable for Eevee to have a more direct Evolution
one in which Eevee genetics would stabilize normally without an external influence?
Also, if Umbreon and Espeon evolve because of Friendship, influenced by day and night,
Then why can't Eevee also evolve naturally, by friendship while under another factor (say the Dream World mayhaps??),
And why can't that evolution be Normal type?
As I see it, Eevee having a Normal-type Evolution is actually more Logical and makes more sense then any of the others.
We just haven't seen it yet.