Yes and no. adb reboot-bootloader is used to get yoi into fastboot mode ard sideload is used to apply updates to a system which can be (and is oftenly) used to overwrite a stock OS. I used sideload for exactly that, my fairphone also delivers a small utility to factory reset every partioin of it using adb and sideload (this is the Windows version from scroll down here). Consider chris stratton's comment. You can look into adb backup and adb restore to only deal with user data and apps. Click me tenderly, click me hard. Pay attention as to apps can opt out of this - explanation in this brilliant answer by Izzy.

Depends but more or less yes. fastboot flashall is typically used to flash the recovery partition only to boot from there and continue with sideload (see 1.).

There definitely are, but I'm not able to do it on my own. The referred utility does exactly that. It should be possible to figure out the exact commands or find a similar utility for your manufacturer. I misused mine to flash everything I needed for me. To do so, i just replaced the respcetive .img files such as boot.img