As you may have already guessed, the correct and best setup would have the Netgear 300N configured to act only as a simple modem. If what you say is correct.....and the Netgear device does not provide a "modem only" option, then there is not much else that can be done in that regard.
If the Guest Network feature is important to you, a better option....if possible....would be to replace the Netgear 300N with a simple ADSL modem. This would allow you configure the AirPort Extreme correctly to provide DHCP and NAT services for the network.
The least desirable option in terms of a correct setup, would be to try to run the network with the Double NAT error.....which can possibly create some unpredictable issues on the network such as slow browsing, intermittent loss of some websites (while others load correctly), and communication errors between any devices that are connected to the AirPort versus devices connected to the Netgear modem/router.
Online gaming consoles hate Double NAT, so it will be difficult, if not impossible to play interactively online if that is a requirement for your network. Likewise, port forwarding....if required by devices on your network....will be destroyed by Double NAT
The bottom line.....sometimes you can get away with a Double NAT error on a simple home network.....and sometimes it is a deal killer. It's just not possible for us to predict how the error might affect your network.
One simple test that you might want to perform to see if the Double NAT settings will "stick".....is to power off the AirPort Extreme by pulling the power cord, wait a minute, then power up the AirPort Extreme again. Sometimes, the AirPort will start back up in Bridge Mode, so you will have to be on the lookout after a power outage. A firmware update can also encourage the AirPort to start up in Bridge Mode if AirPort Utility senses that it would be the correct setup.
We don't recommend that users run their networks with Double NAT if it can be avoided, but the decision is up to you.
