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According to this here Canadian Encyclopedia
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Lemmings breed rapidly and every 3-5 years reach peak densities of 60-125 per hectare. Subsequent population declines involve cessation of breeding and massive die-offs, but are not caused by predators, disease, starvation or stress from overcrowding.
Lemmings seem to be avoiding crises.

More research, more lifeforms, more evidence:
Article 1
Article 2
Big old URL

Very interesting articles which make me have to agree that there are more factors in population control of these species (including rabbits) than just food or predation.
From the article:
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Unquestionably two major variables in populations of different species of small mammals are intraspecific competition and the sensitivity of the reproductive inhibitory mechanisms....

...In any event it seems clear that endocrine-behavioral negative feedback mechanisms operate to increase mortality and decrease reproduction as density increases and can regulate and limit population growth if some other outside limiting factor does not do so first. Even in instances where other limiting factors stop population growth, the internal feedbacks should operate in some proportion to the density achieved.

I am skipping the other portions of the summary and conclusions section for brevity. As you can see the lowering in reproductive rates is related to a hormonal response to the increase in density of the population. This explains why rabbits bred in captivity and kept in separate cages have an unchecked birth rate while rabbits in the wild exhibit a lowering in the birth rate as population density increases. None of the articles state that this is an anticipatory lowering in birth rate. Rather it is a response (lowering birth rate) to a change (increase in population density).
The hormonal changes that are observed to cause the lowering in birth rate, seem to be triggered by the increase in aggressive behavior that occurs in these species as population density increases. This causal link is not directly well supported in the articles but is referenced from other research.
Also note that the authors do not discount the other factors (predation, food supply...etc) in affecting the population or fitness selection. In fact, they all observed those factors in action. Therefore the struggle for food and to avoid death before breeding is still affecting the characteristics of these populations.

Obviously, these species have added reproductive response to increases in population density as a weapon in their arsenal to avoid extinction. So, Darwin had not accounted for all factors when he stated.

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from Descent of Man says
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The enquirer would next come to the important point, whether man tends to increase at so rapid a rate, as to lead to occasional severe struggles for existence; and consequently to beneficial variations, whether in body or mind, being preserved, and injurious ones eliminated. Do the races or species of men, whichever term may be applied, encroach on and replace one another, so that some finally become extinct? We shall see that all these questions, as indeed is obvious in respect to most of them, must be answered in the affirmative, in the same manner as with the lower animals. But the several considerations just referred to may be conveniently deferred for a time: and we will first see how far the bodily structure of man shows traces, more or less plain, of his descent from some lower form. In succeeding chapters the mental powers of man, in comparison with those of the lower animals, will be considered.

I also don't think Darwin took into account that we can control our own breeding habits in response to a future overcrowding potential.

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Beneficial variations are the consequence of severe struggles, thus the varieties of lemming seem to have been unable to evolve. At least the 7 Canadian species - can't expect they'd know as much about the others, can we?
Lemmings have not evolved? There have been no changes in the characteristics within the populations...ever? Is this an observation or a conclusion of yours? What other biologists have observed this lack of evolving or come to the same conclusion?

I did not see that observation or conclusion in any of the articles you referenced.

A faith that connot survive collision with the truth is not worth many regrets. -- Arthur C. Clarke