In the UK crests are awarded to particular named individuals, and are inherited through the male line of descent. Different countries have different rules. They are usually awarded for specific reasons.
So, to prove that you have the right to use a particular crest, you would have to identify a particular person given the right to use it, then prove that you are a legitimate descendant through the male line.
Families have never been in the habit of having crests made upon marriage, unless they knew they were entitled to use it in the first place. William and Kate are not exactly members of the general public in this respect. Or any other respect, for that matter.
There is no such thing as a surname crest where everyone with that name gets to use it. This is a myth which is used by certain companies to sell crests plaques to people who want a nice wall decoration.
Your approach, of documenting your actual lines is quite correct. You never know, you might find something.
There is a board covering family crests somewhere deep in Ancestry's bowels.