Using usb connectors in a non-Standard acceptable method is against the USB specifications. Unless you are looking to conform to the USB specifications, there is no technical problem aside from end user problems. Non-standard usb connectors are fairly common, as is non-standard combinations (Usb female A to Usb Female A is common but against specifications, as are USB Y-Cables used to power external harddrives).
The only thing you can't do, according to the USB forum, is have the usb logo or name.
External power in case of hubs refers to external power adapter (or even a battery). If this is the only power to the hub, and it is a solo source of Vcc for hub controller IC, then it is obvious that downstream ports will have no power. The hub will be simply dead.
The confusion comes in cases when a hub has dual functionality and can derive the power either from external source, or from upstream VBUS, aka "power switching" hub. In accord with specifications, there should be some switch which gracefully replaces external power with VBUS power. When this happens, the hub must turn all downstream ports off. Also in this case all hubs must change the content of their descriptors from being "self-powered" to "bus-powered". Bus-powered hubs have power restrictions, so the host should know this, and apply corresponding power policy during re-enumeration process.
The question, however, is challenging. When VBUS is removed, the hub, as an any USB device, becomes "detached". USB specifications do not define behavior of devices in detached state, see Section 9.1.1.1. So this state is open to interpretations and up to designers.
One interpretation is that when a hub is not connected to bus, it is not attached, and and therefore cannot remain configured. As Section 11.13 says,
If a hub implements power switching, no power is provided to the
downstream facing ports while the hub is not configured.
So, not configured = downstream VBUS is OFF. AFAIK, Microchip follows this interpretation when upstream VBUS is disconnected. There are other hubs with ports that can maintain VBUS, but they are called "charging ports" and are formally outside the USB framework. To support the logic of bus/self-powered switch, these chips have a special pin called LOCAL_PWR.
Hubs that don't use port power switching have VBUS on downstream ports all the time. I believe this is a violation of USB specifications, that's why you can't find the USB-IF certification logo on any cheap tiny hubs.
Best Answer
Yes.
Pretty much. I recall reading somewhere that USB-PD 1.0 was put in the memory hole because RF on the Vbus line could be picked up on an adjacent port making the host think both devices were requesting a voltage increase at the same time. It makes sense but I've been unable to find mention of that to double check.
They do all kinds of crazy things, and some appear to push the limits of sanity and safety. I recall some non-USB protocols putting power on the USB data lines.
I have a charging cable with a USB-A port for the power source and three connections for sinking power. They are micro-B, USB-C, and Apple Lightning. This does not appear to be a passive cable though. It seems to have a small chip in the connector to make sure that the combined load isn't too much for the supply.
That's quite possible. The power negotiation assumes no other power draw on the wire. One method used to detect maximum allowed power draw is to increase the current draw until the voltage sags some amount. If there are two devices and both are looking for a safe level of current then both could be varying their current draw, seeing the voltage go up and down, and never settling on a stable point.
If both devices can drive D+/D- then the signals will almost certainly get scrambled. If only one is connected then we get back to one device negotiating power while there is another device taking a nontrivial amount. The source says "I gots 2.4 amps for you!" The connected sink replies, "Okay, I'm taking all 2.4 amps!" Then the other device sees an open circuit on the data lines, a voltage on Vbus, and so assumes a USB-BC compliant charger and starts to draw power. The voltage sags below the allowed limit and both reset. "I gots 2.4 amps for you!" "Okay, I'm taking all 2.4 amps!" Then a reset from a voltage sag. Then, "I gots 2.4 amps for you!"
USB is built to handle some level of bad behavior so this is not likely to cause permanent damage if something is disconnected after a short period. Leaving it this way overnight could mean the devices switching off and on many times quickly for hours could produce enough heat to damage something permanently.
Given that there's solutions for this that would run in the $15 to $50 range I see no reason to hack up USB cables. Hacking up USB cables in general is a bad idea because there are so many useful and safe cables out there that if there isn't a cable for it then it's nearly certain it would be a bad idea to do it. There's many cables and adapters that violate the spec on the market so even if there is a cable for it that doesn't make it a good idea.