I remember Sanc Aura being bugged in vanilla D2. Did D2r make any changes to how it works?

Does having the aura allow all my damage to ignore the physical resist of undead mobs? Or does just the aura's own damage ignore that resist?

If I have 2 sanc auras active (Azurewrath+Lawbringer) so they stack to create one higher level aura?

It always ignored the physical resistances/immunities of undead, it didn’t add the 300-500% damage to undead that it was supposed to. That got fixed.

The aura acts just like how a fort would against undead only, multiplying your total base damage.

Yes, but it’s mostly unnecessary and something with high base damage will be more rounded for other targets. The best part of the aura is ignoring all physical resistances.

Yeah it's definitely been fixed -- if you take a lawbringer to NM p8 baal runs, you'll notice the huge damage bump to those act2 revive undead mobs, as well as souls and the Act4 undead guys, despite your weapon only having base damage.