“I wonder how “deep” Newman went in that history”
Well since protestants didn’t exist before the Reformation, he didn’t have have to go back too far.
What I gave you included links to historians who did go "further back" more completely than Newman did -- but did not themselves 'cease to be Protestant'.
If you are going to be quoting Newman --- you owe it to yourself to be aware of that one's contemporaries, and their own responses to things such as Newman's 'acorn' developmental theory, which prior to Newman was not taught within [Roman] Catholicism, but needed to be invented to cover the gaping chasm between the Romish claims towards themselves being 'just as Christ and the Apostles established' and all the sundry changes that had crept in over the centuries, with some of those changes having significant theological implication -- resulting in a form of Gospel being preached (in pattern, practice, and demands placed upon supplicants) which was not preached by Paul and the immediate successors to the original 12 Apostles.
Further --there were those who protested prior to the Reformation, surely enough.
In many instances, Romanists slaughtered those complainers, killed them dead, with that activity stretching back through history, at least as far as to Jan Hus & Jerome of Prague (two fine "protesting" Catholics the Romanists Catholics burned at the stake) and a bit further 'back in history' to Wyclif.
Prior to that -- those in opposition to Rome in any regard significant enough to be troublesome to Romish claims to near-unfettered powers over the lives of men, suffered papal crusades (that means war based upon religious difference, in this context) launched against them, with entire towns and the surrounding countryside taken as spoils, then divvied up amongst those who did all the killing in the name of the church of Rome, and that church ecclesiastical community, itself.