Carl was favoured over his cousins the younger sons of the crown prince as they were considered either unhealthy, unintelligent or a bit immoral in their lifestyle.
They were not the only candidates of full Royal blood. Paul and Peter of Greece and Denmark were both considered.
Where the meeting between Juliana and Carl of Sweden was not one to be the basis of a marriage the meeting between Paul and Juliana did go well. Unfortunately Paul's older brother George II did not have any children so needed his brother for the Greek succession. Paul ended up not marrying Juliana but Frederika of Hanover.
Peter of Greece and Denmark had to be dropped from the shortlist because his maternal grandmother had been the daughter of the owners of the Monte Carlo Casino.
There has also been a rumour that prince Carl of Sweden did not try his best with Juliana at the request of his brother-in-law Leopold of Belgium. He wanted his younger brother Charles, count of Flanders to marry Juliana. Even though he ought to have realised that for Queen Wilhelmina a Catholic husband for her daughter was a total no-go.
Some of the Russian princes were looked at too before Wilhelmina caved in and started to look at German princes.
In the end it was not a candidate scouted by the diplomats at the orders of Wilhelmina who married Juliana. Bernhard presented himself and charmed his way into the good books of both Juliana and the Queen.
What has always puzzled me is that neither the Duke of Gloucester nor the Duke of Kent was ever seriously considered as marriage material for Juliana. I know both of them had addiction issues but im not sure their aunt Alice of Albany would have warned her cousin Wilhelmina for them.
Juliana was not much vetted as a marital candidate. The Dutch court was considered the most boring one and Queen Wilhelmina had the reputation of being overbearing. The pitiful example of her father did not help much either. Neither did Juliana's frumpy appearance and preference for older people as conversation partners.
Juliana like her parents and her brother was a victim of the typhoid fever Queen Wilhelmina caught in 1902. One that resulted in her giving birth prematurely to a dead born son.
Had Willem Lodewijk, prince of Orange been born in 1902 the marriage of his parents would have had a much better chance and little sister Juliana might have become Queen of the Hellenes instead of Queen of the Netherlands.
Willem IV would probably have been matched with either one of the Swedish princesses (Ingrid, Margaretha, Martha or Astrid) or one of the many Greek princesses.
Your post basically squares with my posts on the subject. It's not easy to dive right into the royal marriage market, when others regard you with skepticism, even if you're a legally enthroned house and technically their *equals*. As such, the Bernadottes could only gradually dip their toes into the pool. They were lucky to find a princess of Leuchtenberg (as you say, a noblewoman with royal roots).
That house never reigned, but the Beauharnais family (let's not forget also Stephanie, who married the grand duke of Baden) were regarded as royals by virtue of Napoleon's patronage of them. How else could the king of Bavaria (to whom he was an ally) hold his stepson in such high regard, so as to approve a marriage with one of his daughters?
In the next generation, of course, the Bernadottes were able to make fully royal marriages (thanks to Josephine's royal ancestry) -- including a princess of the Netherlands. That's why I find it surprising that Queen Wilhelmina later on initially tried to uphold the highest marital standard possible. It has been said that she herself wouldn't have anything to do with a family which was not an old and reigning one. No mediatized, deposed, or parvenu house would do for her. As it was, her husband was Duke Henry of Mecklenburg-Schwerin.
She strived to maintain the same standard for her own daughter, Juliana -- but struggled: hardly any eligible royal prince was crazy about marrying her. According to an urban legend, a palace secretary advised Queen Wilhelmina that "beggars can't be choosers", and advised Her Majesty to rethink the requirement of "being a member of an old and reigning house", as an acceptable husband for her heir.
After all, the Great War (as World War I was known) had toppled a number of thrones: just about all the European dynasties, both reigning and non-reigning, had no choice but to begin lowering their marital standards. The Netherlands was no exception -- especially since hardly anybody was crazy about being a prince consort, especially to the plain and dowdy Juliana.
It has been said that Prince Carl of Sweden (namesake son of Prince Carl of Sweden and Princess Ingeborg of Denmark) would have been the best bet, highly favored by Wilhelmina. But by then, the Bernadottes had become a respected royal family, for the queen to overcome any objections she may have harbored earlier in life.
In the end, there is nothing like retaining a reigning status, to raise the stock of a house: look at Monaco and Liechtenstein, as perfect examples. Unfortunately in this case, the meeting between the Swedish prince and the Dutch heiress (at the 1934 wedding of the Duke of Kent and Princess Marine of Greece and Denmark) didn't go too well ...