Indeed, 40MHz is for 11n (and 11ac) only. It has no effect for a/b/g.
This being said, I would not recommend using 40MHz in your network unless you live deep in the woods by yourself with no competing wifi networks. The 802.11 standard states that in case of interference between two 11n wifi network, a 40MHz AP WILL
revert to 20MHz. Although the higher speeds can be appealing, the troubles aren't. Also, by using 40MHz, you loose a channel, so when you need to deploy multiple APs, you're limited to two channels (1 and 11). Even though 40MHz doesn't do anything for B/G, it's still in the same frequency space. As you build your network, you'll have to consider all clients and their supported protocols and the amount of channels you can use. In 2.4GHz, you need all three!
As I was working on a setup for an MTCWE lab, my (lab) wifi network (using 40MHz) would become horribly slow when running speed tests. Of course, my house has many APs with a couple of SSIDs. And then, there are neighbors! The clients would go down and unregister. That's because the AP would revert to 20MHz, kicking out the STAs and forcing them to register again. As the link was unstable, registering was problematic at best. Also, good pratices indicate that you should put more APs at lower output powers. In 2.4GHz at 20MHz, you'll have your 3 channels.
I've made a decision to NOT use B/G at home, having a greenfield environment of 802.11n ONLY
. So older clients can't use wifi anymore. But that's my decision. As you may know, a legacy environment (support for older clients) will force your APs to work in protected mode, which will slow your network down. Also, older protocols mean slower speeds, which translates into more airtime for those older clients, leaving less airtime for the newer ones (which get slowed down). Other reasons to drop older protocols if you can do it
As for auto channels, I'm personaly against anything I don't control. If you have a large setup, one AP might notice a neighbor on channel 6 and switch over to channel 4, for example. Now you have 1 and 4 overlapping, causing interference between each other. You can't add a layer of metal that's grounded on your house
so stick with 1-6-11 and plan around that. If your neighbor has an AP on channel 6, put channel 11 on yours. Also, play with power output. Stronger is NOT
As for the original question for your Panasonics, I just don't get it. Old unsupported options??? But you say your drivers have been updated. I have an old Dell Inspiron 15, and it would connect "WHEN" I offered B/G. Have you disabled lower speeds (basic and supported rates)?
Let me know if you put your finger on it.