That was probably one reason, but by no means the only one, nor even IMO a very significant one (and the article you link to, which is a good one from a good historian whose entire nuclear secrecy blog is worth reading, does not make the claim you are making--it gives a number of justifications that were made at the time, and the one you give is not one of them).
The Gar Alperovitz book referenced in the article is also worth reading, as is another historical study, Racing the Enemy [1] by Hasegawa. The latter book is not solely about the decision to use the bomb, but more generally about the process by which the war with Japan ended, but that decision and the process that produced it of course play a large role.
[1] https://www.amazon.com/Racing-Enemy-Stalin-Truman-Surrender/...
> it wasn't even certain at the time that this would work as a bomb
AFAIK there was no doubt that the implosion method used in the Nagasaki bomb would work after the Trinity test. And there was never any doubt that the gun-type method used in the Hiroshima bomb would work--they didn't even bother to test it before the Hiroshima bombing. The only question was what the practical yield would be under bombing conditions. But that could have been assessed by bombing tests on uninhabited locations, as was done after the war.
> it was never as clear cut and about swift endings and saving casualities as came to be believed.
People forget that atomic bomb or not the US was already committed to levelling all cities within Japan.
These things are quite true. They do not, however, mean that wanting to field test two different bomb designs was a significant factor. Based on my reading I don't actually think it was one at the political level (what the military people thought was another matter, but the key decisions were made at the political level). Politically, I think the biggest factors involved were uncertainty about what it would actually take to get Japan to surrender, and the desire (at least once Truman came into office) to keep the Soviet Union from playing any part in postwar Japan, and more generally to deter them from expanding further.
The imminent USSR invasion from the north played a significant part in the calculus of Japan's surrender. Too often the use of nuclear weapons alone gets credit, but there was a more complex political/diplomatic context surrounding _why_ they worked in the case.
Links for anyone interested in reading more on the topic: https://www.amazon.com/Racing-Enemy-Stalin-Truman-Surrender/... http://archive.boston.com/bostonglobe/ideas/articles/2011/08... http://foreignpolicy.com/2013/05/30/the-bomb-didnt-beat-japa...
All this is true, but it's also true that the nukes weren't the only factor involved. A good case can be made that it was actually the Soviet invasion of Manchuria, which started at midnight local time on the same morning that Nagasaki was bombed, that gave the peace faction in the Japanese government enough leverage to get the Emperor to intercede. The Japanese still weren't exactly sure what the nuclear bombs were, and they had already been firebombed for months so having two more cities incinerated was not the huge change that later US propaganda made it out to be. But the Japanese had been trying for many months to get Stalin and the USSR to broker a peace agreement, and Stalin and Molotov had been stringing them along without any real intention of helping, to ensure that Stalin would have time to enter the war against Japan. The Soviet invasion of Manchuria shocked the Japanese government into realizing their true position and made surrender an urgent priority since Japan greatly preferred being occupied by the US to being occupied by the USSR. The nuclear bombs were a convenient way for the Emperor to save face and not have to admit that it was really the strong desire of the Japanese to surrender to the US and not to the USSR that drove the decision.
See the excellent book Racing the Enemy by Hasegawa for a detailed and thorough exposition:
https://www.amazon.com/Racing-Enemy-Stalin-Truman-Surrender/...