Is the lack of success in marriage after repeated Istikhara due to watching forbidden things – despite not committing fornication out of fear of punishment and illness – and considering this a form of "deception," or does the verse "And let those who find not [the means for] marriage keep themselves chaste until Allah enriches them from His bounty" indicate that a lack of chastity may be a reason for marriage not being facilitated, which would explain the current situation?

One must hasten to repent from sins immediately, for Allah Almighty says: "And whatever strikes you of disaster - it is because of what your hands have earned, and He pardons much." Sins can be a cause for punishment, but it is not necessarily so.

As for Istikhara (seeking guidance from Allah), it is entrusting the matter to Allah so that He may choose what is best, and whatever occurs thereafter is the hoped-for good. Therefore, proceed with the marriage and increase in supplication and Istikhara.

You have done well in being cautious of falling into Zina (fornication/adultery) seeking Allah's pleasure. As for the verse in Surah (definition) An-Nur, it commands those unable to marry to be patient and refrain from unlawful acts until Allah facilitates their matter.