https://dl.khadas.com/Hardware/VIM1/Datasheet/S905X_Datasheet V0.3 20170314publicversion-Wesion.pdf
Page 26, you can see that UART B and C live on I2C_A (The RPI's "ID" pins) and "PCM_DOUT/DIN" (Pins 19 and 21 on Le Potato)
Shows the definition of the UART's in question in the GXL dtsi,
shows where the A0 UART is enabled, it should be very similar for UART B and C
There are the pins that need referenced in the same manner as the AO one does.
If you're up for some "fun with device tree", or are quite creative that should do the job. I haven't tried it personally, but you have access to the pins and they have those alternate functions.
As to power management, I haven't used the 4.18 kernel to any real effect yet, so I can't speak to it's consumption, but I would guess some more "fun with device tree" in both u-boot and linux could disable all the stuff you're not using. Also make sure you're using "conservative" or similar for your CPU governor.
If you can't get as low as you want, there are "SoM-style" H3 or H2+ boards that could also fit your needs like the NanoPi Neo/Core/Duo, or even the Libre Computer Tritium H2/3, if you want to keep the flexibility of maintaining the ports. If you search the forums you'll find a ton of "minimize consumption on H3" information.
I guess you came across his first post: https://forum.armbian.com/topic/2419-armbian-for-amlogic-s905-and-s905x/
Once you have multiboot, you can backup the eMMC (especially the dtb), then copy Balbes armbian on eMMC and now you can tweak it how you like it.
eMMC needs many partitions, I don't know why.
I don't know and don't care so much about the u-Boot as long as it works as expected.
Balbes images are based on armbian 5.59 and Kernel 4.18 https://yadi.sk/d/pHxaRAs-tZiei/5.59/20180829
And as I wrote before, with Multiboot you can boot from every source obviously which might be useful doing it the hardway ;-)