I seriously have no clue how is author just gonna write out of this
Well, it could be that Mabel finds some ancient elven stuff to heal Angel... he might just fit the profile of being "the strongest sword" afterwards. The main problem is that nasty dogma dictating a Hero, be it male or female, needis to seduce three companions of the opposite sex. Hard to circumvent.
If we exclude non-fictional options like those you mentioned, we are still in a fantasy world. Magic exists. Gods too, I mean there is a so called Saintess... and we got longlived races like Elves. So we have room for bullshit.
One solution would be to get Liz to lose the qualifications as the hero. How? Death, for example. If there is a way to resurrect her afterwards. Pregnancy, probably... if that's possible while she is "on duty" (= Demon King not resealed)... or maybe the Hero title can be transferred under certain conditions nobody of the short lived races knew.
One thing irks me - Heroes are chosen among candidates (hero bloodline), and likely based on their potential/strength. Liz is of the hero's bloodline, so at least one of her parents must be, too. If it was her mother then Angel as Liz' uncle would be of that very bloodline as well. If so, he was the likely candidate, because he was powerful enough to be called immortal or reincarnator or somehing like that. Someone who doesn't give up and always come back alive. Basically -> hero material.
But Angel was injured by the time the hero got chosen, so he was past his peak and it fell on Liz. Let's say Angel can be healed to full power (or beyond), then it might be possible to transfer the hero role to him. That would solve some things but brings the question regarding Angel's female companions (harem) - Mabel is a given, and Liz - it depends on whether she is strong enough at that point to still be a candidate or not... leaves the third one. Likely a Saintess from hell know where.
We'll see what the author will serve us.