Hi,

So there's no way to disable 11b entirely - you just disable using the 11b, and you set the beacon/management traffic rate to be an OFDM rate. I forget how, but it's configurable. The device still receives 11b, and it can transmit 11b - you just don't let it. (You can disable receiving 11b CCK on the 11n parts, but the driver always configures it in 2GHz mode.)

Short/long guard interval is an 11n thing. It's to do with how much time between OFDM symbols. The reason for it is that the symbol rate is quite high and you dont want OFDM symbols bleeding over into the next symbol in a large environment with reflections. Since you want to do outdoor work, leave short-guard off.

40MHz in 2GHz is pretty risky - 2GHz is pretty congested, and with 40MHz on it'll wait until both halves of the 40MHz channel is free before it transmits. (Note: that's mostly configurable, but doing so will break regulatory requirements.) The 11ac parts are smarter - you can configure separate rate tables for 20, 40 and 80MHz availability and if only 20MHz is available at that point, it'll just use 20MHz.

Always enable RX-STBC. It gives you a little more SNR. TX-STBC won't do anything on a single antenna as it requires two antennas (space time block coding) to work.

DSSS-CCK-40? I think that's "transmit CCK frames duplicated on 40MHz channels for legacy interoperability." That's mostly for legacy interop - eg if you want to transmit beacons, RTS/CTS, etc, then it allows it to appear on both the primary and extension channel so legacy clients can hear the 20MHz side management frames. It should be on by default when you're doing 40MHz 11n.