Where to start? Red triangle and codes

I would look into the C1241 first. It doesn't always mean the 12 volt battery (because the code is about the voltage arriving at the skid control ECU, so all the wiring and connectors on the path between the battery and that ECU could also be suspects). But a quick check of the 12 V battery voltage is an easy call when you've been given this code.

If that battery shows without a good charge, you might give it a good full charge, then clear your codes and see what comes back. You've already posted the codes here, so clearing them won't risk losiing important clues.

 

Still C1241 after "they checked the battery and it's fine"?

That means it's time to break out the multimeter and the wiring diagrams (more info) and track down where the voltage drop is happening on the way to the skid control ECU.

 

I would be guided by what the skid control ECU is telling you, by way of the trouble codes it has given, and I would not be too eager to shoot the messenger. Most often when a car's ECU gives a trouble code, it doesn't mean the ECU is bad, it's just doing its job to tell you about whatever the trouble code means.

Do you still have that C1241 code? If so, something is inadequate about the power reaching that ECU. You say they've ruled out the battery, so it's now a matter of going over the rest of that power path that they haven't ruled out yet.

There is more on troubleshooting C1241 in this post (just ignore the stuff there about other codes).

If you have other brake system codes too, but one of them is C1241, I would still be inclined to focus on that one first. Get adequate power coming to the brake system, then see what it says about any other codes.

Edit: speaking of 'actuator' and 'ECU', it looks like in that post I just linked, I talked about the 'actuator' having a 'system 1' and 'system 2', when I meant the ECU. Oops.

 

Everybody says that because those are the two electrical components they've heard of. (Outside of wires and connectors, which also are worth checking.)

There's rarely much point in trying to leap out ahead of diagnosis. The wiring diagram will show everything (including relays and fuses) involved in that path. Using a multimeter to trace along that path from where the power is to where the power isn't, eventually you find the one place the power arrives and doesn't leave. If that's a relay or fuse, then you've found it. If it's something else, you've still found it.