Then again, historically people were converted before they even knew what most of the mitzvot were.
That may be, but they always had to have the full intention of accepting the yoke of following all 613 post-conversion, as they became familiarized with them. They could not decide pre-conversion that even one of the 613 was too hard and would not maintain it; otherwise the conversion would never have been effective (even with their going into the mikva and verbalizing any statements that they never intended to follow in practice.)
And a gentile is obligated to fully maintain all 7 noachide laws (at penalty of death.) Failure to keep even one of them (i.e. immorality, petty theft, idolatry, etc.) constitutes them forfeiting an afterlife.
An interesting question is whether a Gentile who routinely violated a Noachide Law, or even mistakenly violated such a law on one occasion, can repent from that sin and how he would have to go about repenting.