not typically done within OpenWrt.
https://openwrt.org/docs/guide-user/installation/openwrt_x86#installation
Not always necessary or desirable. It would be good to know what it is that you want to achieve with a larger partition. But, for resizing, see this (which could largely be scripted, if you want):
OpenWrt runs by default with just a single user (root) and doesn't usually use a chroot type construct. What additional setup do you need to do and why do you need to do it with chroot?
Usually not recommended. Typically, backups are of the config files (and other files you may specify). Since you can easily include/exclude packages relative to a typical default installation (using the image builder), the "whole image" is rarely necessary. What is the purpose of this backup -- why the whole installation?