The following is a skeleton form of answers to your very honest questions. Each could be expanded into a book length and has been but I'm skimming at 30,000ft.
1. The collating was largely done before Erasmus. Erasmus was the first one who put the entire Greek NT canon to print. So the answer really is that until then they were still hand copying manuscripts. Therefore it is the invention of the printing press when manually transmitting the text ceased.
2a. It could and has been argued that the process [of collation] was frozen prior to 1516 and Erasmus mostly "assembled" things rather than deciding on anything.
2b. We could also ask this question as to the books in the Canon. Why was the process "frozen" in the 4th Century A.D.?
3. Because of the great uniformity of the manuscripts and their wide use in the churches for more than 1,300 years.
See E. F. Hills
https://www.amazon.com/Text-Time-Reformed-Testament-Criticism-ebook/dp/B07DB7ZBLC
See John "Dean" Burgon
https://www.amazon.com/Revision-Revised-Dean-William-Burgon/dp/1888328010 especially pgs 312-316
4. Doctrinal error and tampering. The Alexandrian text family evidences Gnostic tampering. The Patristics don't cite them not do other churchmen since. The Arian controversy also plays into this as they used Alexandrian texts to support their heretical doctrines.
Sudden chaos overtook Alexandria in 318. A riot broke out and people streamed into the street chanting, “There was a time when Christ was not!” Historical Background The slogan expressed an idea that had become popular: that Christ was a created being. But that idea was opposed by another group…
credomag.com
5. Use in the Churches is the PRIMARY evidence of Providence. God has always preserved His Word for His people. The WCF and LBC 1689 both cite Matt 5:18 (not one jot or tittle shall perish) as a proof text for the assertion of preservation in 1:8.