Concerning the short..
If the port is protected, it should switch off power. But how are you certain you can catch the correct max current in time? If a port has some output caps, you'll measure their short circuit current spike. If the port protection is anything 'fast' (which you force it into because of the a dead short), then the DMM won't respond in time.
Likewise, a gross overload for various durations can also kill power. Many ports are protected with polyfuses. E.g. these might burst in 0,1s with 5A load, but take 1min to burst at 1A load, or 15min at 800mA. If the rating is 500mA indefinite then 800mA or 1A is still too much.
Trial and error tests, in particular short test IMO is bad advice borderline malicious, as it is not going to give any valuable information besides anecdotal evidence, but has high risks damaging poorly protected ports. I would expect a modern Asus MB to have proper protection, but anyone finding this thread through google to test on their 15 year old system is going to blow it up.
That Owon scope looks very weird. Why is it using an USB A plug for a downstream port? It should be USB B or C.. and if they need more power then either A) use a second USB A plug upstream or B) use a DC barrel jack of some sort.
Those PSU to USB A, or USB A to USB A cables also look downright dangerous..