All we KNOW, actually KNOW, is that those traits were THERE in the population and their alleles were there.
Wrong. We don't KNOW that, you are just
assuming that. You are doing exactly what you are accusing evolutionists of doing, i.e. making unwarranted assumptions. In the days that there were no bulldogs and no one had ever seen a dog with as short a muzzle as that of a bulldog, there was no way anyone could know that there existed an allele (or alleles) for such an appearance.
There are two reasons why this should be so. First, genetic science had not yet advanced to the level of today, meaning that no one knew even what an allele was, let alone that they knew whether a particular allele existed or not. Second, before there were bulldogs, the allele(s) for the form of its muzzle really did not exist, it's as simple as that.
You simply cannot know that such an allele has always existed, because it is impossible to know it, and because it hasn't. In your own words: "it's all assumption".
On the other hand, we
do know that mutations occur, we can see them happening all the time, we can even
make them happen, and we have a record of them happening throughout life's history. It is only a logical conclusion that they must play a role in how evolutionary history unfolds. True, in the end, the role of mutations is an assumption, born from logical inference, but it's testable assumption, and all tests to date have vindicated it.
"Ignorance more frequently begets confidence than does knowledge: it is those who know little, not those who know much, who so positively assert that this or that problem will never be solved by science." - Charles Darwin.
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