1)Could LOS be the problem? I know I cannot shoot critters just down, outside a murder hole. And are you subtracting the range, correctly. Isn't shooting/viewing distance shorter the higher up the entity is? Unlike in reality. What is he looking thru, fortified walls or upward gates? Not sure if it matters, I use fortified walls to prevent too much exposure.
2)For that reason, 'unlimited Raise Dead ranges' and a few others, undead movement speeds, daywalking undead, etc.. I no longer play default worldgens. Default worldgen is for rogue players. Adv worldgen is for us. It takes a few tweaks but can still be enjoyable in 34.xx, IMO. Don't give up, just try other embarks.
Got a game going now. Playing [34.11] on my end, but not seeing any snatchers/thieves passing restrained war dogs. Three war dogs, one by the back door, and two by the main entrance bridge, either side. The 2 have overlapping ranges, but not so far they wander out on the bridge.
I've never used piggies, but I've used kitties in a pinch. Finding it hard myself, as well as you, to understand stealth in DF. I uncover a cloaker, and he tip toes right out over a yard full of traps. *scratch head* It makes no sense that they should bypass traps, while uncloaked on their exit/fleeing path, they should suffer the same as others at that time. Take care, Knutor