I just purchased a BXT Gravel 135 frame, to build up as a winter frame to save my primary bike from the salt, or to put on the trainer in the summer, or let my wife ride, etc. It will be my first bare frame build.

I’m doing a bit of a mock assembly now to make sure I have all the parts and whatnot and with the headset is an integrated headset with some internal routing that allows you to use a normal stem. But unless I’m missing something, the spot where the cables route into the headset is basically just an open hole, you can directly see the top bearing.  Is this right? Seems like a terrible design. Especially since replacing that bearing involves cutting and re-running brake hoses through the frame. Is there a better option for this style of headset, that isn’t really expensive? Or am I just assembling wrong?

Certainly interesting if they can get a product to market and throw some good support behind it (support historically for Magene products seems... lacklustre). For example, see this thread:

https://chinertown.com/index.php?topic=4221.0

The P505 certainly seems to have been a success for them though and they've at least shown with that that they can make a reliable product.

How can the same shifters work with both brake cables and hydraulic housing?

A clear pass to me. Not worth the risk and hazzle.
I wish Tavelo all the best with that pricing policy.
Myself just pulled the trigger for a almost new Scultura 9000 with Alu rims yet full R8100 for less than 2400€. A way better package and value if you ask me.
Now I'm looking forward for some new wheels from the Shanghai bike show and that the Yen comes up in value again and then I'll upgrade. Current favourite by quite some margin is 9Velo with carbon spokes.

I simply put bubble wrap around battery. I rode 100 km with no issues. I am still looking for a permanent solution.

I can now confirm that the SL7 Di2 battery mount works for the 268 
Can't put the link in, so have added a screenshot: