Electronic – nother way to think of/draw this simple diagram

In normal circuit analysis, we almost never think about which way the electrons are flowing. We nearly always calculate and visualize how the "conventional current" is flowing.

In the case of a diode (to simplify things somewhat---see Steven's answer for some special cases), conventional current flows through the diode from the anode to the cathode; that is, conventional current flows in the direction that the "arrow" of the diode symbol is pointing.

1. For a black-box type of converter like these linear regulator replacements, black-box empirical analysis of the transfer functions is about all you'll be able to do. Gain-phase analyzers are expensive but allow you to directly measure things like loop stability, plant response, input impedance and audio susceptibility. I wouldn't advocate $8k or more to characterize a 3-terminal regulator. You've already hit on one of the potential gotchas - lack of headroom caused by the input sagging under load.

2. With buck converters, paralleling involves some external OR-ing circuitry (be it a diode or a MOSFET) to prevent backfeed. Also, if load sharing is required, some form of balancing (forced current sharing or droop sharing) is also needed. Adding diodes will work but there's always the possibility that there will be no load sharing until one of the converters starts dropping out. I doubt you'd be able to make these 3-terminal devices share well given the low current rating without wasting lots of power implementing external droop (and even then, variations in the OR-ing diode drop could swamp out everything).