Like I said, I had 9 devices attached to the 2.4G when I did that download speed test yesterday
<lol> sorry about that, I missed that little update.
Let's put a stake in the ground, I'm going to say 50 to 75 Mbps for "N" over 2.4 GHz is reasonable to typical depending on distance. "G" devices will be slower and "B" devices significantly slower. These old devices can slow down new devices some, just being there will slow them down some but I don't know by how much. (Now the really smart guys will give you what you can actually expect : -)
(Just for grins I placed my old [old as in it's not even AC] laptop about 15 ' from my router and ran Internet speed tests. My Internet service is only 100 Mbps so my testing is limited. I ran N over 2.4 GHz and achieved 80 Mbps. I then ran N over 5 GHz and achieved 118 Mbps.)
I know you said you played with channel settings but load up one of those free WiFi analyzers. It will tell you what channels your neighbors are using. From that you can determine the best channel or ... you may find there just ain't any good options.
I still like your idea of trying "N only" You don't have to leave it that way, you just want to know if it makes a difference ... to start with. (That setting should dump any "B" devices that might be holding you back.)
You could also try shutting off the furthest (from the router) devices. They could be pulling your speeds down. (Again, not forever, just to test.) Or, turn them all off and turn them on one by one until things start getting crappy. (Just one naughty device can impact everyone.)
Also, do take a look at your traffic monitor for 2.4 GHz.
If other devices (e.g., cameras) are generating a lot of traffic then you're going to be limited to just your fair share.