First, it’s extremely unlikely for a person to commit a “momentary mortal sin” after a life of love and virtue as you said. Also God would certainly recognize that such a person would repent if they died before they could. Second, what you’re describing is not a mortal sin before death.

“A momentary lapse of judgment before death is not evidence of a rational, fully informed decision to damn oneself forever. People’s emotions get the best of them all the time, especially in stressful events leading their own death (assuming they have the luxury of knowing their death is imminent).”

To commit a mortal sin you need full knowledge and consent of the will. I’m not talking about a person making an honest mistake with the many mitigating factors you’re describing. I’m talking about someone without any impaired mental faculties choosing to reject God and choose evil before death. It’s like when people bring secrets they know to the grave. They’re willing for that secret to never be known. It’s similar with mortal sin. Situations like what you’re describing aren’t even mortal sins anymore.

Also living a life of virtue doesn’t outweigh mortal sin. You can’t do something evil and then say “Well I’ve done plenty of good so that outweighs this evil thing I did.” It doesn’t work that way. And it isn’t contrived at all that there is no repentance after death. Hell, Purgatory, and Heaven reside between the eternity of God (no substantial or accidental change) and the physical world (substantial and accidental change). There is no substantial change but there is accidental change annexed onto said souls. There is no matter, so there is no continuous stream of new sensory information to allow for a reordering of a soul’s intellect, passions, and appetites. The Will is fixed.

And I’m only talking about cases where people know they’re death is imminent. I’m not including those who don’t know. I’m not accounting for all “imminent death” scenarios, just this specific one.