Home Network Setup Advice Please

AP mode on the routers is a quick and dirty way to enable.... AP mode - turns off  pretty well all the features and makes tem layer 2 devices.. But, since you went through the excersise already, and disabled what needed to be disabled, you should be good to go. Except you probably do not have NTP working on the NG routers, as you would use the WAN port on the routers, if you used the AP mode selection on the routers, which is buggy with some of the NG routers, so, you would end up using a LAN port, worst case, anyway. 

There is no way to disable NAT on these routers..  If you are using different SSID's, that is probably the way to go, as I don't think any of these routers have an min RSSI setting, and there is no proprietary means to kick wireless client off of AP's that are sticking. They don't support 802.11.r/k/v, and alot of wireless clients don't work well with 802.1/r anyway.  I'm not sure if they have  a wirless TX feature to tweak your TX ouput, whihch is  key when trying to set up a wireless environment, to get wireless clients to correctly roam. If you want to check the manuals to your routers, go here.  

EDIT: Don't use auto channels ever. These routers are smart enough to pick the correct channel. They usually end up pick an overlapping 2.4ghz channels. Use 1,6,1 for 2.4ghz. You can call SSID anything you rwant. But, adding the _ext on the outdoor APisn't doing anything, as extenders do that for different reasons. 

EDIT2: If you want to get real experimental with your AP's (using one SSID for 5ghz and one SSID for 2.4gz, you could try one channel for 2.4ghz, and one channel for 5ghz, and see if your clients roam. Other vendor's tried this technique in the early days for Zero Hand Off (aka  roaming)