1. Let's clear this up for you. There are two kinds of current flow, conventional flow and electron flow. Electrons have a negative charge. But in circuits it is just math, and in math there are "conventions". It doesnt matter what the convetion is as long as it stays the same.
In conventional current flow, it goes from + to -.
In electron flow, it goes from - to +.
Conventional flow is what is used since it's been around longer and things don't like to change. But we know electron flow exists. But what is conventional flow then? Conventional flow is also called "hole" flow. A hole does not have + charge like a proton, but is rather the absence of a negative charge. When an electron moves from one area to the next, it leaves behind an empty opening called a hole. If you visualize an infinitely row of electrons with one gap moving, and each electron is moving left by hopping into the gap (and therefore leaving a gap behind it for the next electon to hop into), the gap will appear as if it is moving to the right while the elctrons are moving to the left. This gap is the hole and therefore moves in the opposite direction of the electrons.
2. To account for a how a power supply behaves when it cannot provide the current required, you have to examine how the power supply itself works rather than just a little symbol on the page. In most cases (all actually that I know of) the power supply will either
-be destroyed due to overheating
-or it's output voltage will drop as it tries to supply more current. THe voltage dropping will reduce the current that the load is drawing due to I=V/R. This will continue until I and V balance out reach steady state. Remember that during the transition period, the currents and voltages cannot change instantly. They all move smoothly from one point to the next (even if it's very very fast). What this means is that the current being draw doesn't instantly rise up to I=Vinitial/R so that there is a single instant in time before the Vinitial starts to drop where the equation does not hold. I must go up smoothly towards I=Vinitial/R, and as it reaches the current limit from the power supply, V will start to drop which will slow down the rise of I until V and I reach equilibrium.
What this means is that if you have an ideal voltage source and two parallel resistors, the current will work itself out. If you have an ideal current source instead of a voltage source, the voltage across the resistors will work itself out based on the current in the resistors. If you have a non-ideal voltage source, due to the internal workings of the power supply which will vary and is more complicated than the two resistors, the voltage will being to drop as the current exceeds the maximum that can be supplied. The act of dropping the voltage will cause the current to drop in turn due to I=V/R.
other authors are incorrectly teaching just because they can visualize/explain it better?
No, they are teaching it like that so YOU can visualize it and understand it better. Have you ever heard the saying, teaching is the act of lying a bit less each time? Otherwise you would start out with quantum mechanics rather than V=IR.