Well as for the maximum time before dhuhr juma'ah can be prayed. Another fatwa on islamqa reveals that the beginning time for jumu'ah is the beginning time for the 'Id prayer:
Imam Ahmad ibn Hanbal and a number of other scholars are of the view that the beginning of its time is the beginning of the time for Eid prayer; as for the time when the sun passes its zenith, that is the beginning of the time when it becomes obligatory to hasten to do it. (source islamqa #140222)
Some evidences used for that are the ahadith in sunan an-Nasa-i, sunan abi Dawod and sahih al-Bukhari beside the hadith from sahih Muslim. The hadith from sahih Muslim is used by some of the hanbali scholars like al-Khirqi الخرقي to declare that the beginning time is (in one view or narration) the fifth hour or (in the other one) the sixth (the hours are quoted in the hadith I'll quote later) as quoted in al-Mubdi' fi sharhi al-Muqni' (see here) of ibn al-Muflih ابن المفلح. Both the fifth or sixth hour refer to a timing which is close to noon according the view of the hanbalis. And only this can be concluded from the ahadith used as evidences if one interprets the hours as staring from the beginning of the day.
However the majority is of the opinion that the ahadith referring to the prayer before the noon are referring to the call or takbeer of the prayer not the prayer itself.
Al-Bahuti البهوتي in his kashf al-Qina' 'an matni al-Iqna' somehow corrected or relativized the above statement saying that the textual beginning of the time is that of the Eid prayer which was adapted by Ahmad as the Friday is the Eid of the Muslims as quoted in the hadith in sunan ibn Majah and al-Muwatta'. He quoted a hadith/narration on the authority of 'Abdullah ibn Saydan which imam Ahmad and ad-Daraqutni -in his sunan- (I couldn't find it in either) have quoted and used as evidence saying that he prayed the Jumu'ah with Abu Bakr (including Khutba and prayer) before the noon and 'Omar (including Khutba and prayer) ending with the noon and with 'Othman who ended it after the noon. He also quoted other sahaba who have done so without giving any source. The scholars of the majority oppose this saying that first ibn Saydan was unknown and secondly those who identfied him as a tabi'i say nobody know about his trustworthiness (see for example in ibn Hajar al-'Asqalani's fath al-Bari). Ibn Battal ابن بطال al-Maliki in his commentary of sahih al-Bukhari attributed the permission to pray jumu'ah at the beginning time for Eid prayer to Mujahid and said imam Ahmad held the opinion that it is permissible to pray it before the noon.
It is also rather recommended to pray jumua'h close to dhohr time or afterwards as this is the majority view. This is the major statement of the fatwa quoted above and also the view of a majority of scholars among the hanbali school of fiqh who said it is better to pray it after the beginning of dhohr time or close to it to leave the dispute (difference of scholars) aside according the book of ibn al-Muflih quoted above.
You may find this quoted in the book called al-Insaf fi ma'arifati al-Khilaf of the hanbali scholar 'Ala' ad-Dyn al-Mirdawi (or al-Mardawi) علاء الدين المرداوي (Vol. 1 p 375-376), where while discussing the karaha of travelling on Friday's after the noon (the obligatory time for jumua'ah as stated above) quoted a statement of imam Ahmad saying (My own translation take it carefully):
ويشترط لصحة الجمعة أربعة شروط . أحدها : الوقت ، وأوله : أول وقت صلاة العيد
And for a jumua'h (prayer) to be correct(valid) there are four condition one of them is the time. and it's beginning is the time for the Eid prayer.
Note that the beginning time of the Eid prayer is the time one is allowed to pray after the sunrise so it declares the end of the time for karaha for prayers (however the prayers which are makrooh at this time are in first place any optional prayers).
In fact that going early to jumu'ah is recommended due to ahadith such as:
"Any person who takes a bath on Friday like the bath of Janaba and then goes for the prayer (in the first hour i.e. early), it is as if he had sacrificed a camel (in Allah's cause); and whoever goes in the second hour it is as if he had sacrificed a cow; and whoever goes in the third hour, then it is as if he had sacrificed a horned ram; and if one goes in the fourth hour, then it is as if he had sacrificed a hen; and whoever goes in the fifth hour then it is as if he had offered an egg. When the Imam comes out (i.e. starts delivering the Khutba), the angels present themselves to listen to the Khutba."
(See sahih al-Bukhari, sahih Muslim, al-Muwatta', sunan an-Nasa-i, sunan abi Dawod and Jami' at-Tirmidhi)
Imam Ahmad and a-Shafi'i and a majority interpreted these six hours as separate timings and an-Nawawi -in his commentary on sahih Muslim- therefore said that the first hour is at the beginning of the day, while imam Malik and others consider these hours as different small time gaps beginning from the noon.
Other scholars (even among the madhhab) considered the earliest time for jumu'ah to be somewhere between "when the sun has risen to the height of a spear (which is the time for the earliest of the two 'Eid prayers) or in the fifth hour or sixth hour." as stated in the fatwa you've pointed at in your post. They also added as an evidence that one can only pray qada' for jumu'ah once the time of dhohr has entered as the qada' of jumu'ah is the dhohr prayer.
Conclusion:
So basically the evidence for the permission to pray jumu'ah before noon or dhohr time is based on a (rather weak) narration of a tabi'i who witnessed it with three among the rahidi Caliphs and which was compiled by imam Ahmad (where?) and ad-Daraqutni beside other statements of sahaba who have practiced it this way.
