What was Prof. Brand's plan?

Question

Having rewatched "Interstellar" (2014), I realized that I don't understand the original plan.

We learn that Prof. Brand never intended to go through with Plan A, as he was convinced that it cannot be done. Instead, he played that card simply to convince people to work for the preservation of the species, while believing that they are working to save themselves and their loved ones.

So, originally, our heroes were supposed to find a habitable planet and return to instruct NASA where to send people, only to discover that it couldn't be done. None of the crew knew that Plan A is a sham (as Murph put it).

Sure, Dr. Mann knew it, but the chances of finding him alive were slim (1/3 just to pick his planet, further reduced by great chances of death during some 35 or so Earth years alone on an unknown planet), and him "handling" our heroes was not something to be counted on.

So, had it all gone as planned, our heroes would've returned, without deploying the "population bomb", bringing only the information about a newly found and confirmed habitable planet. Then that bomb would still have to be sent back to that planet, along with the crew to take care of it.

This would've brought us back to the original problem (people having to work to save the species, instead of themselves and their loved ones), with the addition of a new problem: Endurance was the last ship capable of such a trip (this was mentioned by Prof. Brand when he was introducing Cooper to the whole "NASA still exists" thing). Further, the fuel from Endurance would've been spent, the ship itself would have had billions of kilometres behind it (requiring repairs they had no way to perform). So... how would've this worked?

Note that I am assuming the original plan worked as intended:

1. A habitable planet was found without sabotages or major problems (i.e., Endurance would've made it back, worn out but in more or less one piece), and

2. No new ways for space travel were discovered while our heroes were having fun in space.

Answer 1

Plan A relied on one part you are forgetting. The formula. Plan A depends on the Senior Brand "figuring out" the formula for gravity, which would allow the space stations, like the NASA building, to lift off Earth, with a self sufficient population, thus saving humanity. Coop and the junior Brand's part was finding the planet to which humanity would head towards. The Plan was for everybody to follow.

                 COOPER
      So if we find a new home, what then?

                PROFESSOR BRAND
      That’s the long shot. There’s Plan
      A and there’s Plan B. Did you
      notice anything strange about the
      launch chamber ...

 INT. LAUNCH FACILITY - MOMENTS LATER

 Cooper cocks his head, puzzling at the VAST chamber ...
 there are structures built SIDEWAYS around the CURVED walls...

                COOPER
      This whole facility ... it’s a
      vehicle? A space station?

                PROFESSOR BRAND
      Both. We’ve been working on it, and
      others like it for twenty-five
      years. Plan A.

                COOPER
      How does it get off the Earth?

                PROFESSOR BRAND
      Those first gravitational anomalies
      changed everything - suddenly we
      knew that harnessing gravity was
      real. So I started working on the
      theory - and we start building this
      station.

                COOPER
      But you haven’t solved it, yet.

                 PROFESSOR BRAND
      That’s why there’s a Plan B.

The population bomb was only Plan B. It had no use in Plan A. Professor Brand told Coop to trust him that the formula was almost complete.

                BRAND
      The problem is gravity. How to get
      a viable amount of human life off
      this planet. This is one way - Plan
      B. A population bomb. Almost five
      thousand fertilized eggs, weighing
      in at under 900 kilos.

                COOPER
      How could you raise them?

                BRAND
      With equipment on board we incubate
      the first ten. After that, with
      surrogacy, the growth becomes
      exponential - within thirty years
      we might have a colony of hundreds.
      The real difficulty of colonization
      is genetic diversity,
           (Indicates vials.)
      This takes care of that.

Cooper looks at the equipment. Unenthusiastic.

                COOPER
      We just give up on the people here?

                PROFESSOR BRAND
      That’s why Plan A’s a lot more fun.

Furthermore:

 INT. PROFESSOR BRAND’S OFFICE
 Professor Brand watches Cooper as he gazes over the vast
 tracts of ALGEBRA covering every available surface.

                COOPER
      Where have you got to?

                PROFESSOR BRAND
      Almost there.

                COOPER
      Almost? You’re asking me to hang
      everything on an ’almost’?

                PROFESSOR BRAND
      I’m asking you to trust me.

              PROFESSOR BRAND
      Find us a new home. When you
      return, I’ll have solved the
      problem of gravity. You have my
      word.

You are right, they only find out about the three planets after going though the wormhole. Kinda. They get detailed info. NASA already had a general idea before sending them to that system:

                COOPER
      Because you don’t have resources to
      visit all twelve.

                DOYLE
      No. Data transmission back through
      the wormhole is rudimentary, simple
      binary ’pings’ on an annual basis
      to give some clue as to which
      worlds have potential. One system
      shows promise.

                COOPER
      One? Kind of a long shot.

                BRAND
      One system with three potential
      worlds ... no long shot.

So they knew that A) multiple planets with B) a thumbs up. There was no expectation that Coop and co would be stuck in a time slippage and loose decades. This was a last ditch effort anyway.

Of course, assume that they found a planet in 10 years and came back.

This is what you want to understand, or guess at as neither the draft or final scripts tackle it. Had they found a good planet and came back without any problems, without dropping the population bomb, what then? They go back with more people and drop the bomb then. The 10+ years isn't a problem, and with a guaranteed known good planet, they have more incentive to send people and expend the effort. Brand could even keep the lie going, saying that they need to send a colonization prep team ahead to prepare for the larger colony.

Besides, fuel and supplies aside, had the plan gone perfectly, the Endurance space ship could return and go back many times. Space travel isn't a problem for space ships. It's landing and takeoffs in atmosphere that provide the greatest strain. All those miles wouldn't wear it out, it's not a car... They explicitly planned for a return. On earth they could refuel.

Answer 2

This is an interesting question that I don't think the movie fully addresses.

I think the most likely scenario is that Professor Brand was planning on telling the crew of the failure of the gravity equation after the crew arrives at a habitable planet, but before they leave said habitable planet.

This reveal could result in Coop angrily storming back home, but he surely wouldn't force everyone to come with him, and I'm fairly certain at least one of the crew would volunteer to stay back and execute plan B.