1: The reason you couldn't connect may have been to do with the laptop and AP being on different subnets after changing either of their IP addresses or the subnet mask. I usually have a think about what went wrong so I know to avoid it next time. That either results in me realising a way to get straight back in, or realising that I have indeed locked myself out. If the latter is true, then I reset the device and configure it from scratch. (Note: resetting won't unbrick a bricked device, but having been very careful when flashing firmware, I have never bricked a device so far.)
2: Yes. Using the same SSID and passphrase for APs on the same LAN (and with the same WAN/default gateway) allows portable devices to roam near-seamlessly between cells, which is very useful and convenient. For different SSIDs, a device will not automatically switch unless it completely looses signal from its current association, and then it will take a few seconds to reconnect. This is presumably because there is no way of knowing for different SSIDs that they are on the same LAN, so open connections could be lost if the device was to switch to an SSID with stronger signal. In my experience, using the same SSID allows devices to switch cells more freely, with only about a seconds interruption when the switch occurs. (However, for dual band APs I currently see no advantage in using the same SSID across both bands of the same AP because dual-band devices strangely seem to prefer the 2.4GHz channels in the SSID which are generally more cluttered.) At the very least I recommend to experiment with it.
3: Ignoring the obsolete 11b standard, the 2.4GHz channels should ideally be one of 1, 5, 9, or 13 (in most of the world). Annoyingly this AP (with stock firmware version
2.00.10; latest at the time of this writing) doesn't have a ‘2.4GHz (G+N)’ option in the ‘Band’ field (and is not supported by OpenWrt), but I have no 11b devices anyway (and have never) so I still use channel spacing as for only 11g and 11n. I use my range extender in AP mode connected by internal Ethernet wiring to the router, or via a powerline connection if I put it in a room without an Ethernet port. I do not recommend relaying via WiFi because each frame is transmitted twice and the whole concept doesn't scale well for additional repeaters, at least not without proper manual configuration or advanced mesh networking protocols. If performance and reliability is desired and Ethernet wiring is not appropriate, powerline technology should in most cases be used to connect the AP to the router. In AP mode connected via Ethernet (or powerline), I recommend using separate 20MHz-wide channels. If however, reliability and performance is less important and you are using it as a repeater, then by the nature of the hardware having only one 2.4GHz tuner, it has to be on the same channel as the SSID that it is repeating, thus significantly increasing the contention on the channel. Note that using a 40MHz-wide channel is not a remedy for contention, and may actually suffer increased contention by colliding with traffic from neighbours. In theory it would also slightly decrease the battery life of small portable devices such as smartphones. So having a WiFi network of separate 20MHz-wide channels and an Ethernet/powerline backbone is the ideal setup.