Hi,
As far as time code sync goes the HDSP will ct as the 4th ADAT machine. So it will be at the end of the line of the ADAT sync cables.
It's been over a decade since I've had to deal with ADAT machines so not sure if I remember all the steps.
Page 24 of the current HDSP9652 manual explains the time code syn setup in Cubase. It is based on any older version of Cubase so the window will look different but the settings are all basically the same.

Technically it is always best to have the RME card be the master clock but it maybe with these older ADAT machines and with the BRC you may need to have it slave to the BRC or first ADAT.

If so then set the 9652 card to Autosync.

The EMU 0404 card has Coaxial SPDIF so you can easily set it up to use for monitoring whats coming in and out of Cubase.
You will not be using it as an audio device in Cubase. You will technically be using it as a stereo AD/DA converter.

You will need 2 coaxial RCA cables running between the 9652 and the 0404. The EMU clock needs to be set to slave to the SPDIF input, The RME card will pass the audio clock from the ADAT to the SPDIF so both the RME and the EMU are slaving to the ADAT machines.
You can then use the EMU mixer to route the SPDIF signal to the analog outputs of the EMU.

In Cubase you just need to use the SPDIF channel as your default Main out your mixes.

You can then take the analog out of the EMU and run it to the Tascam mixer if your not already doing so. This will act as you monitor return from Cubase. You can leave the ADAT machines hooked and use them as needed.
As far as Cubase cares your juts using channel 25/26 of the 9652 card.

Make sure in the HDSP settings panel that SPDIF IN is set to coaxial and that SPDIF OUT has nothing checked.

Thanks Chris