I was thinking you could grind out a channel in the concrete and run a liquidtite conduit using THHN wire. It would be noisy and messy and if you do the work yourself, no one would know.
The other options are also viable, and more to code, just fugly. Although you can paint them and run them so they are more out of sight.
I was also thinking you could extend the cold leads and while that may work, it doesn't look ideal. See below.
Don't forget to put in two temperature probes. And possibly a third. I had two fail me on one installation. It still worked, but it only worked on the temp at the thermostat, not the temp in the the floor. A BIG difference.
Good luck with your job. And good choice using Ditra, it also acts as a thermal break so you won't be losing that much heat to the slab. Although, if you put a 1/4" layer of cork, you would be amazed at the difference that just the cork would make.
This is from Schluter's website:
"Extending the heating cable cold lead
The cold lead is made up of two 14 AWG conductors with a copper braided shield, that is used as the grounding conductor. The extension must be made with building wire that is suitable for this application and complies with applicable building and electrical codes. The cold lead itself is not made of building wire and therefore cannot pass through studs unless run through a conduit. Extension of the cold lead requires the addition of a “code compliant” junction box that must be accessible at all times. The maximum length for extending the cold lead is 75 ft (23 m)."