How do we know the noone has reported an aura of strength +2.5?
I always assumed that the measure of aura strength on a scale from +1 to +10 is purely a game term.
Ars Magica generally does not have many pages dedicated to explorations of how characters inside of its world sees the world of Ars Magica. I dont think that people in the ArM universe measure auras on a scale from +1 to +10, probably they mostly dont measure them at all, except by some deeply personal measure caused by the application of InVi magic, such as e.g. the intensity of shivering induced or how spicy the taste of the aura is.
As you suggest aura strengthing is a good example. Imagine for simplicity's sake a magic aura of strength +2 that increases in strength at the solstices to +3. In game terms we would say that the change is a sudden jump of +1. But again this could be a reflection of the reality of the characters or it could be a game mechanical simplification. Looked at from the perspective of the characters it is possible that around the time of the solstices this aura is slowly building in intensity until it climaxes at the solstice and then slowly comes down to its normal level again.
The same goes for vis.
There is nothing to say that vis is not naturally divisible beyond the pawn, only that hermetic magic has no good way to consistently interact with and measure such. how would a magus know that the pawn they just used was actually 1.12 pawn instead of 1.0? It is hinted in RoP:M that auras contain/are made of "fluid vis" which would be divisible on much smaller scales that "pawned" vis. Likewise it is hinted that "pawned" vis occurs because the fluid vis concentrates, but does it concentrate instantaneously or a little at a time? There are canonical vis sources implying both are true.
There are good reasons why the rules are as they are, since it would be a nightmare to do the maths based on equations for magical phenomena. But IMO the underlying metaphysics make more sense that way and to a sufficient degree that it makes for better stories too.