I've asked #ICOM staff the #IC705 questions at #hamRadioFriedrichshafen in different years which were never answered. But at #UKWTagung a few minutes ago, someone from ICOM did provide answers 👍.
Paraphrased from memory:
Q: Will SD card content be made available via USB? - A: Not sure, but maybe the firmware memory is already full.
Q: The manual stipulates to leave the battery attached even if on external power. Why? A: Needed if the external power is glitchy; the CPU doesn't like glitches.