Minwax Polycrylic warns that it may amber if
>applied over a light-colored oil stain. Is this true?
Yes. Everything yellows with time. If you need to reduce this, go with a water base and don't even use oils or oil-based products beneath it. On pine though, I'd expect the darkening of the timber under sunlight to be more obvious than the change in a thin coat of a good modern varnish.