Following up on this, and to respond to the more general question of "Could Kindle web access allow an email hijack?", rather than the specifics of your situation.
One thing I would strongly recommend is that you check that the 'Always use https' setting is enabled. It used to default to disabled, but now defaults to enabled. If you signed up to GMail when the default was for it to be disabled, I don't know if it would have been changed. There is more information, including instructions on how to set it, at
this GMail help page.
Enabling the 'Always use https' setting means that all data sent between your Kindle (or PC, laptop, whatever) and GMail is encrypted, which makes it much more difficult for anyone to eavesdrop on your connection, whether you connect via wi-fi, 3G or something else.