> why, in all of these tube theory sources, do they not adhere to this?
Real tube heads always follow the current top to bottom. Current is like water. Any fool knows that water flows downhill.
> I've found only one source that describes a tube's current flow from plate to cathode.
You need to find more tube-heads.
> The rest describe current flowing from cathode to plate.
That's the direction the electrons flow. Since a normally-operating vacuum tube only flows one way, we can ignore the +/- sign on current. Since the current is all electrons, they do flow cathode to plate. Occasionally it is useful to see it that way. But to make the math come out right, including polarity, you need to note it as negative current.
> "conventional current" is just a theoretical construct
No, it assumes that current is carried on positive charges and on negative charges denoted with a "-". At the time, they knew that potential had two sexes that looked the same but weren't. To make the math work, they had to put + and - signs in, and arbitrarily picked one to call "+".
In 99% of situations, you can't really assign one polarity or the other: charges flow both ways. Positive one way, negative the other way. This is true of wires and semiconductors. In wires, while you could say the electrons move, each time one moves right you find a positive ion behind it, with a net flow of positive charge to the left. In semiconductors the distinction is even slimmer. In gas tubes the positive charge flow is very real. It is only in high-vacuum tubes that we can actually believe in electrons. And since they are assigned a negative polarity, the current DOES flow from plate to cathode when the electrons flow cathode to plate.
At least in vacuum tubes, the reality is unambiguous, no matter which way the author describes it. When transistors came out with electrons and holes flowing in a solid, and PNP and NPN, it got real confusing.
