First, could you find no way to write the code so that you didn't need to adjust things by -1px all over? I don't know the situation, but it has a code smell.
Second, I should think you'd be able to import them. They don't seem large enough to need separate sheets anyway, but importing is something you'd do if for example some pages needed to load that css and others didn't.
The @import line has to come first in whichever main stylesheet is in the <link> tag.
So if you have
<link rel="stylesheet" type="text/css" href="main.css" >
then in that main.css sheet
the first line should be your @import webkit1.css (or whatever)
inside the webkit1.css/whatever stylesheet, you have the @media lines like you do now.
Another option is to list the webkit sheets in <link>s like you are with the main.css. You can list media and I believe @media queries in that <link> statement.