To each fall, because D&D doesn't do physics well
There is no provision in the rule for multiple falls per turn, so the rule is applied the same to each. The scenario given will (using that optional rule) go as follows:
After the first Warlock initiates the chain, it will follow these looping steps:
Bob starts a (new) fall from one platform.
As per that rule, when he starts falling he descends up to 500 feet.
Bob lands on the platform 400 feet below. The fall is over.
Next Warlock casts eldritch blast and pushes Bob off the platform
This repeats until we run out of Warlocks/Platforms, one of the Warlocks miss, or Bob dies and is a corpse rather than a creature (Repelling Blast requires a creature). At no point does the turn end. This is dumb. It is important to mention that D&D is not a physics simulator and edge cases (hah!) like this are where the DM is expected to use their common sense. A more rigorous rule might be:1
At the end of each turn or when a creature starts falling, a creature that would fall descends until it lands or has fallen 500 feet since the start of its last turn.
However this rule is also likely to cause some problem in a specifically constructed scenario, though I haven't figured that one out yet.
It's worth emphasizing that this is an optional rule provided as a DM tool (as is my proposed one) and it might be more useful to think of it as how falling could be approximated inside of the initiative system and not a replacement of a DM's judgement.
1: If you think this rule is a bit convoluted, you may have found the reason the rule in XGtE doesn't try to be rigorous.
They would not fall, but flying speed would be reduced to 0
The Fly spell reads:
The target gains a flying speed of 60 feet for the Duration.
Since flying speed is a speed, This means that when speed is reduced to 0, flying speed is also reduced to 0. The rules on flying speeds being reduced to 0 read:
If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell.
(emphasis mine)
The flying rules say that when flying speed is reduced to 0 the flying creature falls. However, since these rules essentially say that if the flying status is created by magic, with the fly spell as an example, in this case the character would not fall, so they would simply be unable to move after using the cunning action during this turn.
Best Answer
This is more advice than a hard-and-fast answer, because there isn't a way to tell by RAW when exactly in a turn that something happens; if you knew for certain that, say, the barbarian lost their flight ability one second before the end of the turn, you could calculate it, but that "one second" figure really doesn't exist. So the final answer will have to be a DM call. I'd go one of two ways:
Both of these assume that the one making the call is the DM.