1. It is only in the heretic janamsakhis of the Minas* that we find first mention of Bhai Bala.
It is ridiculous to suggest that Minas created the character of Bhai Bala to denigrate Guru Nanak but then created such a foundational and vastly popular Janamsakhi of Guru Nanak through him as the following introductory paragraph in the book "Atals...Travels of Guru Nanak" suggests:
The Bhai Bala Janamsakhi is the best known as also the most popular of all the Janamsakhi traditions. It surpasses all corresponding traditions in the graphic description of the early life of Guru Nanak, for instance, with regard to his love for his sister Nanki, his bethrothal, his marriage, his parent-in-laws utter dismay after the Guru resigns his appointment at Sultanpur and decides to become an udasi or Self-absorbed traveller. The style of writing is conversational. Another remarkable feature of the Janamsakhi is that the Guru has been shown here travelling not as ordinary mortals would, by traversing the land assiduously, but by miraculous flights from one place to an other.
And certainly miraculous flights would elevate Nanak to divine heights rather than denigrate him.
Rightly, a little below even the writer at sikhiwiki.org coul not resist appreciating the same Janamsakhi by Minas created fictitious character:
Still, Bhai Bala Janamsakhi covers very important aspects of Guru Nanak's life. It covers the Travels did by Guru Nanak. It has various sakhis which we do not find in other sakhis and which really happened.
Or as an entry on an other page: This janamsakhi has had an immense influence over determining what is generally accepted as the authoritative account of Guru Nanak Dev Ji’s life. Throughout the nineteenth century the authority of the Bala version was unchallenged. An important work based on the Bahi Bala janam-sakhi is Santokh Singh’s Gur Nanak Purkash commonly known as Nanak Parkash. Its lengthy sequel, Suraj Parkash carries the acount up to the tenth Guru and contains a higher proportion of historical fact, this was completed in 1844.2.The language used in this janamsakhi was not spoken at the time of Guru Nanak or Guru Angad, but was developed at least a hundred years later. Some of the hymns ascribed to Nanak are not his but those of the second and fifth Gurus.
This proves nothing. Popular literary works are always edited even again and again by later generations to make them more readable by the general public of the times, sometimes merely on the advice of publishers. Some of the later editors may just be the men of the publishers who may have added certain popular hymns to make the Janamsakhi more inclusive and saleable on the advice of the publishers.
3. At several places expressions which gained currency only during the lifetime of the last Guru, Guru Gobind Singh (1666-1708), are used e.g "Waheguru ji ki Fateh**." Bala's janamsakhi is certainly not a contemporary account; at best it was written in the early part of the 18th Century.
The response to the point 2 holds good for this point as well. The expressions "Waheguru Ji Ka Khalsa" and the like had become quite popular with Sikhs during and after Guru Gobind Singh and the later writers or their publishers may have included them to exploit their popularity and emotional appeal among Sikhs.
*Minas (meaning those people who look different from outside than inside and harbour ill will against some other persons or community from inside, or in their minds) was the name given to those who bifurcated from mainline Sikhs from the times of fifth Guru, Guru Arjun Dev. They were said to be the followers of Pirthi Chand, the eldest son of fourth Guru, Guru Ramdas and elder brother of fifth Guru, who had a grudge against fifth Guru as the later was given the Guruship overlooking his claim by his father.
**"Waheguru ji ka Khalsa (see below) Waheguru ji ki fateh" is a way of greetings for Sikhs now called khalsa as ordained by the tenth Guru, Guru Gobind Singh. It literally means "The Khalsa belongs to God, (Its) Victory belongs to God."
Khalsa, the pure ones, was an other name collectively given to Sikhs when, following the martyrdom of Guru Teg Bahadur, the ninth Guru and father of Guru Gobind Singh at the hands of Muslim ruler of Delhi, Guru Gobind Singh converted them to fighter-Sikhs or saint-soldiers from merely meditating Sikhs whose main aim then became to secure justice for the underpriveleged by any means and even by fighting if need be and who were ready to lay down their lives for this at all times. The spirituo-philosophical thought behind this name is that since Khalsa had then surrendered to God they will be left with no individual selves and hence no individual impurities or weaknesses and will thus act/fight for all practical purposes like God Himself for the just cause. (Above are two pictures of the same one person Baba Deep Singh, the one as a saint and the other as a soldier.)
The wise say there are two ways to know God. Either "to surrender" or "to enquire" Who am I? In both the cases one gets rid of one's small self and realises one's oneness with greatest Self or God. It is like a drop of water in a sea on surrendering or on enquiring finding that it is in fact itself the sea and thus realising its oneness with the sea. So the purely spiritual aspect behind the name Khalsa will be to enable Khalsa to realise God - verily the final job of any real Guru and here of Guru Gobind Singh.