I can only attempt at an answer for the first part of your question: though likely to have developed earlier, the practice of treating churches as "sanctuaries" was noted by Augustine as something "novel" (de cit. dei. I.7) and remarkable during the barbarian sacking of Rome in 410, where "the largest churches were chosen and set apart for the purpose of being filled with the people to whom quarter was given, and that in them none were slain”. He draws attention to the extraordinary fact that the barbarians spared those who took refuge in certain churches or Christian sacred spaces (I.2, I.4), and that this was without precedent in warfare vis a vis pagan temples.

Adding to this, the strongly pacifistic streak in early Christianity (which e.g. disapproved strongly of Christians in the army, cf Tertullian de. cor. mil.) would have either outright forbidden, or at least express disdain for, the presence of weapons in a church, or, most of all, violence taking place there.