It is simple to guarantee the lowest price. All you have to do is say that "IF" someone can find a lower price advertised elsewhere you will beat it by x%

This way you are guaranteed to have the lowest price.

I can't do that. Prices of my products are fixed by the manufacturer and it's impossible to change them for anyone retailing the products.

What I have is more products from more manufacturers so I have manufacturers that offer lower prices than their competitors. Most of my competitors deal with say companies A, B, and C. I deal with A,B and C as well as D E and F.

You can't use superlatives unless they're supported somehow by a 3rd party source on your landing page (at least, that's the way it used to work) Sometimes you can get away with doing it for a while before they catch on (and if you ARE doing it against policy, you can almost guarantee one of your competitors will probably bust you)

In a case such as you outline, I'd probably try to find a way to say something like "Nobody works harder to bring you the best prices on the market" Only a lot shorter to fit the format.

If I did a survey of say the top 20 results in Google myself and indicated that, would that keep google happy?

They seem to be approving the ads right now so it's not a problem just yet. But I'd like to keep this in my ads I think, seems like perhaps a competitive advantage.

Prices of my products are fixed by the manufacturer and it's impossible to change them for anyone retailing the products.

I think you would still be ok to make the claim because if what you are saying is true then no-one will ever be able to submit to you a lower advertised price, since there will be none.

I have seen big name retail stores do this with "exclusive" brands.

They claim they will beat any other price but they are the only one who sells that brand... guaranteeing that this will never happen.