You don't need to edit the Device Button mapping for each device except maybe the Sound and Display devices since the "One" is missing the Sound and Picture Buttons. Editing the Device Button mapping is very tedious since by default it's pre-configured with all the commands for that brand. The device mode should be used only for testing, troubleshooting or accessing seldom used commands. You can map any command from any participating device to any activity key using the Activity > Customize Buttons.

However, for each device, you should test all the commands paying attention to their behaviour (toggling/cycling vs discrete action) to find easter egg commands (like discrete power and input commands) that are not on the original remote. Armed with this info, configure the device power and input settings. Use the discrete method if you found the discrete commands. Then create your activities making sure you select the correct device and its input for each function (volume control, tuner, etc). The activity, by default, will assign the Volume keys to the sound device, the channel keys to the tuner device, etc. Finlay, check and/or modify the Activity > Customize Buttons.

You will find the Harmony programming interface quite infantile. It was designed for every one making it annoying for power user but you will get used to it. Just don't be afraid to experiment and try all the options/features so you know where they are in case you need to access one of them in the future. In general, all the device parameters are found under Device > Settings. The "Number of Repeats" (now nameless) is found under Device > Settings > Troubleshoot > "xx responds to some commands either too many times or only occasionally".