1. bangapro is the local APN we use with our operator, instead of it you should use your own APN from your operator.
2. Yes, it will reset to default LAN, to change it with uci:
uci set network.lan.ipaddr='192.168.0.1'
3. uci for PIN:
4. Full config might wary from devices and current settings set, so the best way to know what you can set-up in your device is to open up full uci list by yourself:
And you can use pipe to search for something through the list:
uci show | grep keyword
Regarding your question about APN in the dropdown menu - yes, these are APNs available to you, but sometimes APN you need isn't in the list so you have to fill it by yourself, just use "--custom--" selection and write your APN there.