So, I operate a 7520 under OpenWrt as VDSL2 bridged modem and it draws 6 Wh/h, see here:
Ideally yes, but FritzOS does not really allow that anymore... so either PPPoE passthrough with no PPPOE credentials configured in the fritzbox, or some homebrew solution to enable bridge mode somehow.
No, only if the Fritzbox handles PPPoE, but then you likely will have double NAT.