ad 1) Freetz-NG is a completely different project and unrelated to OpenWrt. I don't understand what you mean with "supported method"? Supported for what?
ad 2) Backup of the configuration or the entire system software? Assuming you mean the entire system, there is no need, see 3) below. Assuming you mean the configuration: Can't this be done from the vendor firmware?
ad 3) All AVM devices have a recovery program that works pretty well (download from AVM website). All that needs to be working is the bootloader. If the bootloader is broken, a backup doesn't help either unless you have a way to flash the ICs directly.
Keep in mind that the 7530 does not have support for the DSL modem with the current OpenWrt release. You need an unofficial build which includes DSL support if you want/need that. I run it as a dailyldriver and it works well.
Yes, it's a modification of the AVM firmware. I've mostly used it for the tools to extract the DSL firmware blob.
Yes you're right again I was just re-reading the unsupported DSL modem function, I will think about if just install frettz or just extract the DSL firmware.