FS85 can be waked up in power-down mode even if VBOS is off. So I think it is ok for you to use power-down mode as the TLF35584 standby mode.
As far as we know, we have several customers using FS85 power AURIX TC2xx or TC3xx and the systems work well.
FS85 do not have HVLDO and all the regulators are off in standby mode or power down mode.
Do you know our VR5510 family SBC which integrated with HVLDO? The other regulators are very similar with FS85, but we still suggest to you to use FS8510 to power Aurix processors as most customer use this SBC to do that .