First of all those test results are odd. Even a Brother with 1000 Ranged Defense should get hit by 5% of all shots, so there is that.
Also the observation of “the shield never eats the arrow” probably results from that as well. You get an animation for the arrow hitting the shield instead of a miss quite often when trying to shoot “through” someone with a shield. Its probably just that the 15-30 Ranged Defense for the shield don’t matter against the 1000 the Brother already has.
I can confirm that if you target someone behind someone with a shield, the shield eats the hit quite often. Shooting center mass into a bunch of enemies seems to check 2 things.
1. If the intended shot is a hit.
2.1 If it is, the game checks if it is still a hit against the guy who causes the shooter to suffer from 75% “interception penalty”.
2.1.1 If your BS roll is high enough to get a clean hit on the guy intercepting you will get a hit on him
2.1.2 If your BS roll is high enough to hit but not high enough to account for shield defense bonus, you will get a hit on the shield.
2.2 The shot doesn’t hit and scatters
2.2.1 The shot scatters to an empty field and nothing happens
2.2.2 The shot scatters to an occupied field and hits automatically
This makes a lot of sense as it means you can reliably hit more often by aiming center mass into a group of opponents without shields.
The best way to make a shot go away is by “catching” it with a shield.
From these observations I came up with the following and run it successfully on Expert Ironman.
Fighting against opponents who hang back to shoot you:
Frontline put exactly zero points Ranged defense and gets Heavy Armour, Battle Forged as well as Kite Shields – they want to get “hit” as much as possible. Goal is to catch as many arrows as possible with the shield’s Range Defense bonus.
Second line (Archers, Polearms) skills Ranged Defense and Anticipation.
The front line then puts up a Shieldwall to get to 50+ Ranged Defense with adjacency bonus. Anything other than Mercenaries and Master Archers is down to 5% hit chance a
Crossbows camping in the back aren’t able to shoot back with their 6 tile range. So if your opponent camps in the back his Crossbows are of no concern for now. Bows without Bow Mastery can only hit your Frontline and get mostly caught in your Shieldwall. The occasional freak miss hitting someone will happen anyway but rarely.
Be aware of high ground – if you can, take it. If an opponent with multiple Crossbows has a height advantage against you, I’d strongly consider retreating and reengaging. Even a significantly weaker opponent gets scary with high ground.
After setting up go for exposed targets without shields. Use Aimed Shot to make those Arrows count. Creating a breach to shoot the opposing Crossbows/Archers has priority.
Fair warning, fighting like this makes the game a lot more manageable until you have higher levels and better armour, but it takes ages. Its a real challenge on your patience to maneuver to high ground and then Shieldwall/Skip turn with 9 Brothers just to shoot 3 arrows a turn. But its fairly safe and completely worth it if you have a tough fight ahead.