I find it do simple even a 3 year old can understand it.
1) Imam Ali (as) refrained from calling the people as it would lead to a bigger evil.
By, bigger evil you mean the Sahaba apostating and starting the idol worship again, as you mention in your earlier posts. The evidence for this claim is the alleged "SPECIAL ILM" of Ali(ra), and it seems that you have inherited some of it, due to which you came know about the excuse of Ali(RA).
But the claim you made from the "special ilm" you inherited goes against Quran, because we don't find in Quran that people turning away from Islam or people apostating would harm Islam in anyway. On the contrary Allah informs us that if people turn away from Islam or turn back from following some commands of Islam, the Allah says that He will replace such people with better people.
We read:
O you who believe!
whoever from among you turns back from his religion, then Allah will bring a people, He shall love them and they shall love Him, lowly before the believers, mighty against the unbelievers, they shall strive hard in Allah's way and shall not fear the censure of any censurer; this is Allah's Face, He gives it to whom He pleases, and Allah is Ample-giving, Knowing.[5:54].
Behold! you are those who are called upon to spend in Allah's way, but among you are those who are niggardly, and whoever is niggardly is niggardly against his own soul; and Allah is Self-sufficient and you have need (of Him), and
if you turn back He will bring in your place another people, then they will not be like you.[47:38]
If you do not go forth, He will chastise you with a painful chastisement and
bring in your place a people other than you, and you will do Him no harm; and Allah has power over all things.[9:39]
But
if you turn back, then indeed I have delivered to you the message with which I have been sent to you, and my Lord will bring another people in your place, and you cannot do Him any harm; surely my Lord is the Preserver of all things.[11:57].
As for Protection of Islam, then Islam was already destined to be dominant, whether people apostate or not, if they would have apostated then Allah would replace them with better people, yet Islam would remain dominant.
He it is Who hath sent His messenger with the guidance and the religion of truth, that
He may make it conqueror of all religion however much idolaters may be averse. [61:9]
In commentary for this verse, Imam Ahmad recorded from Tamim Ad-Dari that he said, "I heard the Messenger of Allah saying:
«لَيَبْلُغَنَّ هَذَا الْأَمْرُ مَا بَلَغَ اللَّيْلُ وَالنَّهَارُ، وَلَا يَتْرُكُ اللهُ بَيْتَ مَدَرٍ وَلَا وَبَرٍ إِلَّا أَدْخَلَهُ هَذَا الدِّينَ، يُعِزُّ عَزِيزًا وَيُذِلُّ ذَلِيلًا، عِزًّا يُعِزُّ اللهُ بِهِ الْإِسْلَامَ وَذُلًّا يُذِلُّ اللهُ بِهِ الْكُفْر»
(This matter (Islam) will keep spreading as far as the night and day reach, until Allah will not leave a house made of mud or hair, but will make this religion enter it, while bringing might to a mighty person (a Muslim) and humiliation to a disgraced person (who rejects Islam). Might with which Allah elevates Islam (and its people) and disgrace with which Allah humiliates disbelief (and its people).) Tamim Ad-Dari ﴿who was a Christian before Islam﴾ used to say, "I have come to know the meaning of this Hadith in my own people. Those who became Muslims among them acquired goodness, honor and might. Disgrace, humiliation and Jizyah befell those who remained disbelievers. [Tafseer ibn Katheer]
Now the Question to you is that, why didn't Ali(ra) let the evil people who corrupted Islam get replaced with better people, as Allah(swt) promised? Like Haroon(AS), He should have implemented the obligation of Amr bil Maroof and Nahi al Munkar, and if the evil people would have left Islam, Allah would have replaced them with better people.
NOTE: We Sunnis believe that certain people did apostate after Prophet(SAWS), and the Sunni Caliph Abu Bakr(Ra) did believe in the promise Allah made, hence Allah did fulfill his promise.