Yes the HG2 is slightly over-rated. All three 18A cells can do 20A, but their internal resistance is higher than the true 20A cells, so they will sag more at this discharge rate and it's really at the limit for them.
Some tests list the HG2 as true 20A but my own measurements clearly showed the difference in IR vs the LG HE4, so there's no possible magic here, it's not the same discharge rate to me.
And why are the higher amps more conducive for single battery or unregulated vapes while the 2 battery would do better with a lower amp?
Because at 50W you are already super close to the max discharge rate of these cells. In fact if you go the full 75W these single cell mods can offer, you should be using one of the very few 30A CDR cells instead (like LG HB6)
Those using the Tubo at 70W know the dreaded "weak battery" message when they use the 30Q or the HG2. It's because the voltage sag under load is so high that it triggers the mod protection.
In a multi-cell mod, be it a series or parallel configuration, the load is spread and each cell has less work to provide. But if you were aiming at the crazy high (read 140W++) powers, you would face the same issue at some point. Thankfully we never need that much in our applications so that's a non issue.
And lastly, does amp pulse, i.e. 35 pulse, mean the same as maximum continuous with cutoff?
Pulse ratings are completely arbitrary as there's no agreed standard. So the figures are basically useless. Pulse for how long? With what recovery time? It's impossible to compare and it's quite misleading (and dangerous) in practice. Only good for marketing and fooling users.
And I also believe it is safer to have less amp per battery in a multi-battery mod, because the amps will combine? Whereas in a single you only have the one battery's amps to rely on?
In series voltages combine, so effectively a 10A load at 3.7V will become a 5A load at 7.4V in a 2S configuration. It's a bit more complex in a parallel configuration and one should be more careful with proper pairing and not exceeding the CDR.
I think he gets most of this research from mooch? Who steered me clear of hcigar
Mooch does an excellent job and is the most well known, but he's not alone. I try to never rely on a single information source and I also perform tests on my own from time to time.