Salam! It is not unusual for the period to be absent for a month, or several months (without being pregnant) and have inconsistent periods.
This can indicate that something is wrong, or the girl/woman is perfectly fine.
peace

Is that opinion shared by any scholars?

Eeeeeeh, no...............

654 And if you are unsure of those women whose menstruation has seized, the waiting period is three months, as well as for those whose menstruation is absent. And those who are pregnant shall wait until birth......

How in the world did young girls get involved in this verse?

Because it says "as well as for those whose menstruation is absent" ?

How do you do Salah using the Quran Alone?

Does anyone who takes the Quran as their only source of guidance do any rakah in their salah?

If so how do you know how many to do?

Quran 654 "And those who no longer expect menstruation among your women - if you doubt, then their period is three months, and those who have not menstruated. And for those who are pregnant, their term is until they give birth. And whoever fears Allah - He will make for him of his matter ease."

Tafsir Al-Jalalayn

And those of your women who (read all? or all?i in both instances) no longer expect to menstruate, if you have any doubts, about their waiting period, their prescribed period shall be three months, and those who have not yet menstruated, because of their young age, their period shall be three months ? both cases apply to other than those whose spouses have died; for these their period is prescribed in the verse they shall wait by themselves for four months and ten . And those who are pregnant, their term, the conclusion of their prescribed period if divorced or if their spouses be dead, shall be when they deliver. And whoever fears God, He will make matters ease for him, in this world and in the Hereafter.

Is this verse really talking about the divorce of young girls? What are your thoughts?