Origin and evolution of 'on the bubble' in senses related to 'having an uncertain outcome'

Question 1)

Just a quick hint from the Phrase Finder. The expression apparently originated in the world of car racing in the ‘70s, specifically in the context of Indianapolis 500 race:

On the bubble:

This American expression seems to have originated in the car racing community, in particular the aficionados of the Indianapolis 500 race. The first citation I can find is from a report on the 1970 running of that race, in The Lima News, May 1970:

• "On the 'bubble' is rookie Steve Krisiloff whose 162.448 m.p.h. was the slowest qualifying speed last weekend. With only six spots open, Krisiloff's machine would be ousted if seven cars qualified at a faster speed this week end."

The question is of course, why 'bubble'? The most popular theory relates to the Indy 500 and suggests that if a driver were about to qualify and then someone did a better time and pushed him down the rankings into the non-qualifiers then dreams of qualification would be dashed and his bubble would be burst.

Question 2)

If the car racing theory is correct, the original meaning of the figurative sense of “on the bubble” is the one suggested by the ODO. The expression conveys a sense of uncertainty and its usage may have evolved to the more current and general one as suggested by the Collins Dictionary cited below.

As noted in the Lexmaniac

Its meaning seemed relatively straightforward, but it is used several different ways, most of which seem to coalesce around a state of uncertainty, of not knowing what’s in and what’s out.

Question 3)

As suggested from the following source the expression might come from “sit on the bubble” from which “on the bubble” but, unluckily, it does not provide further details:

on the bubble (of a sports player or team) occupying the last qualifying position in a team or for a tournament, and liable to be replaced by another. North American informal.

This expression comes from sit on the bubble , with the implication that the bubble may burst.

Question 4)

I think the usage and meaning expressed in the New York Times is well defined by the Collins Dictionary:

On the bubble (informal US):

in a situation in which the outcome is uncertain but already in the process of being determined or decided.

also as noted in the Lexmaniac:

“DR. EMMETT C. MURPHY defines “on-the-bubble behavior” as “any behavior that can compromise the achievement of an organization’s mission,” which is even more specialized, not to say idiosyncratic. Well then, why should “on the bubble” mean “on the threshold” or “on the brink” or “waiting to have one’s fate decided”? Apparently no one knows.