This really helps and I don't see it posted much elsewhere. So for speed running the item you need to check for one of the boat entrances into a region before a non-world boss and that the rat mother is there with only 4 babies on her not 5.

If you still need the answer, I might help you to find out. I've got both items resently. Hard to belive but I spend nearly an hour rerolling the adventure, trying to find mother and baby in one map.

I ended up testing this with a friend of mine, this Amulet only applies to yourself. Your teammates does not receive any defense bonus even if they are wearing the same necklace.

I'm convinced that this is almost impossible to get. I've got the Reisum DLC on March 7th, been playing every day since then, at least 2 runs of Reisum per day (even more before). Played the Campaign through twice.
The Mother asked me once (1 time) in a month in Campaign mode. Every other time, which is at least 50+ rerolls +1 Campaign reroll (I think it's over 200 rerolls at least), she either didn't spawn or was just a simple merchant. The Krall baby spawned twice (2 times) in all of this, and not in the Campaigns. Needless to say, never with the Mother present.

The only thing I can tell you is that if you find the Mother, count the kids on her belly, if she has 5, you need to reroll. If you enter Wuthering Keep, the whole map looks pretty different, if the cage with the rat baby spawns.