The ahadith report a vow made by Solomon, that he would sleep with several of his wives in one night, with the various reports numbering them as much as 60, 90, 99 or even 100. In hadith science, when multiple versions of a report having the same narrator chain give different numbers everytime, the understanding is that the number was remembered differently by the narrators because of it being insignificant to the purports of the prophetic saying.
Abu Hurairah is the sole narrator of this hadith and there is no suggestion that he heard it from the Prophet on multiple occasions, meaning the latter cannot be the reason of the contradiction. What might have happened is the prophet spoke on one occasion of Solomon's vow to sleep with an unspecified large number of his wives in one night, as well as the number of wives he had in total. The narrators might have mixed the 2 statements.
I will go round in the night to my ninety wives, and every one of them will give birth to a child (who will grow up) as a horseman and fight in the cause of Allah His companions said to him: Say" Insha' Allah." but he did not say Inshii' Allah. He went round all of them but none of them became pregnant but one, and she gave birth to a premature child. And by Him in Whose hand is the life of Muhammad, if he had said, Insha' Allah (his wives would have given birth to the children who would all have grown up into horsemen and fought in the way of Allah).
The hadith speaks of Solomon desiring to have many male children so as to serve in war for God's cause. He vowed sleeping with a large number of wives for that purpose in a single night, but forgot the angel's inspiring him with expressing his reliance on Allah (other versions mention the angel). Despite his wish, none of the wives he slept with was able to conceive, except for one who gave birth to a premature child that probably died in infancy. The Arabs referred to them as split/divided/half children because of incomplete growth.
Not every failed endeavour is a sign of divine disapproval. Neither does the hadith say that his failure was caused by God so as to punish him. God may either allow the natural flow of causality to follow its course, or alter it as He deems fit. In this case, had Solomon initially relied on God, then God could have rewarded him with success in his objective by altering the course of causality. Practically speaking, most healthy men are able to have sex with a very large number of women in a short time if they interrupt the act with one partner then switch to another. This might have been the reason for Solomon's failure to impregnate his wives.
As to the hadith quoted in some books of tafsir saying that the revelation of sura kahf came much later than the day following the challenge, because the prophet did not utter the phrase "if God wills", presents several internal incoherences, besides it being rejected for having unreliable and unknown narrators in its chain.
It says for instance that once revelation came 15 days later, it contained a rebuke to the prophet. No such reproach exists in sura kahf. Nor are there any traces in any tradition, authentic or else, of the supposed rumors due to the prophet's incapacity to answer the following day as he had promised. Furthermore no link exists between the verse upon which the tafsir and narration are based (18:23-4) and the question asked later on concerning Dhul Qarnayn 18:83.
The verse teaching to never plan anything before relying on God 18:23-4 and upon which the narration is based, is a parenthetical statement inserted in the flow of the discourse about the youths of the cave. It is connected to the previous verse which is about refraining to research into things known only to God, so in the same way one should not make plans for the future without acknowledging that certainty in all matters only belongs to God.
Finally, the narration says sura Kahf came in answer to 3 questions by the Jews (the youths of the cave, the "ruh" and the powerful traveller/Dhul Qarnayn) whereas the sura cites only one question related to one topic, Dhul Qarnayn and then proceeds with the answer.
In contrast there is no link between a question and the story of the youths (a Christian story, irrelevant to the Jews who supposedly asked the question) and neither does the sura respond to the 3rd supposed question about the "ruh". That question was uttered on another occasion and answered in 17:85 which is a much earlier sura than sura kahf.
