Correlations between tool use and evolution
This is a very confusing question! You have many sentences that end in question marks but then you answer them yourself, ending with a final question mark which is basically "are there references that show I'm right" but which is phrased in a way that makes the answer rather trivial (yes, of course our ancestors show a decrease in canine size, I'm not sure how else you would expect to get from there to here. Strength is harder to measure in fossils I think). 

I'll address a few points. First I think this question makes humans to be much more hapless and vulnerable than we are:


  We humans would be nearly defenseless against large predators without weapons.


Humans are large predators. It's a nitpick because your other points about the relative strength of great apes and how we use weapons are correct, but I think it's relevant to the "oh us poor squishy defenseless humans" trope. We tend to think of our size as normal but we're actually among the biggest animals.


  Even small mammals can outrun humans.


Not for long they can't. One hypothesis that seems to have some credence in the field of human evolution today is the Endurance running hypothesis, which points out that almost no other animal shares our capacity to run for a very long time and that hunting stragegies based on this may have been a significant aspect of our evolution, that would explain many aspects of our physiology.

Of course this kind of hunting would likely involve weapons, but as you point out humans and proto-humans have been using tools for a long time.

You seem to have an image of weak proto-humans inventing weapons to turn the tables on the large predators that would eat them, but how would they have survived before? I had a similar one about hairlessness and clothes; probably we tend to think of human bodies as immutable human attributes, and all our tools as optional. When the truth, as you said yourself, is that humans as a species aren't separable from our tools and intelligence; our bodies, minds and behaviors all co-evolved. There never was a weak, naked human with no tools; before tools our ancestors weren't weak or naked and with tools of course we are neither. Also, intelligence and social behavior count for a lot; a single human might be vulnerable to a large predator where a coordinated band of hominids, probably armed with sticks and stones and some understanding of how to use them, would be another story.

But you want actual references and evidence, which is harder. I linked to the Wikipedia page on the evolution of canine size but strength isn't so easily measured in the fossil record. I think this paper might be the closest thing:

Body mass and encephalization in Pleistocene Homo 

Relevant quote from the abstract:


  On the basis of an analysis of 163 individuals, body mass in Pleistocene Homo averaged significantly (about 10%) larger than a representative sample of living humans.


That's assuming body mass is a good proxy for strength, which isn't clear to me.

The following paper confirms that humans are indeed weaker than other apes and that the genes related to skeletal musculature have changed a lot since our common ancestor with them but doesn't give a timeline:

Exceptional Evolutionary Divergence of Human Muscle and Brain Metabolomes Parallels Human Cognitive and Physical Uniqueness

That paper goes with the hypothesis that loss of strength was caused by diverting resources to our brains, but other papers suggest there could be a trade-off between strength and fine motor control, like here:

The strength of great apes and the speed of humans


  More than 50 years ago, Maynard Smith and Savage (1956) showed that the musculoskeletal systems of mammals can be adapted for strength at one extreme and speed at the other but not both. Great apes are adapted for strength--chimpanzees have been shown to be about four times as strong as fit young humans when normalized for body size. The corresponding speed that human limb systems gain at the expense of power is critical for effective human activities such as running, throwing, and manipulation, including tool making. The fossil record can shed light on when the change from power to speed occurred. I outline a hypothesis that suggests that the difference in muscular performance between the two species is caused by chimpanzees having many fewer small motor units than humans, which leads them, in turn, to contract more muscle fibers earlier in any particular task.


(note that this paper mostly seems to be proposing a hypothesis and not demonstrating it, and it isn't very cited... but a lot of the papers citing it look interesting. Including this entry in the "could pre-humans hunt with weapons" column:

Clavicle length, throwing performance and the reconstruction of the Homo erectus shoulder


  These data (...) suggest that the capacity for high speed throwing dates back nearly two million years.


)

I think the question of how human strength evolved in their evolutionary history is probably very answerable with modern genetics, I've been looking for papers along the line of this one, which is about jaw muscles not upper-body strength:

Myosin gene mutation correlates with anatomical changes in the human lineage


  Using the coding sequence for the myosin rod domains as a molecular clock, we estimate that this mutation appeared approximately 2.4 million years ago, predating the appearance of modern human body size and emigration of Homo from Africa. 


but I haven't found any; it's possible this specific question just hasn't been answered yet.