uart-a is distinct from the debug (I think of it as the console) uart, which is known in these chips as uart-ao-a. That AO shows up in other places as well, and it indicates that these devices and GPIO pins are Always On. Many devices and pins can be literally turned off under program control to manage the power budget.
But there’s no reason not to use uart-b or uart-c if you prefer, as long as they don’t require pins that you need for something else. uart-b, for example, uses the same pins that I2C-a would need.
As for your DT issues, I can’t offer any help, as I’ve never used any of the {Rasp,Armb}ian distros. I still can’t find the post I (imagine?) I saw that made it clear which of the ones LC has images for are best supported, but this one makes it clear that at some level there’s Debian and then there’s all the rest, each with its own funny ways.