To simplify things (and to save lifes :)), always assume you will become visible when you shoot. There are very specific cases when you MIGHT remain invisible (being behind multiple bushes, one of which is within 15m and others are outside of 15m) but they are pretty rare and quirky.
BTW, tanks always see other tanks within 50 m regardless of anything - through walls, bushes, mountains, etc.
Camo skill simply decreases the distance from which your (hiding, i.e non-shooting) tank will be seen. IIRC, similar to Repair skill, 100% camo crew provides 25% bonus. So, if some tank, in certain situation, would have detected you from 400 m, your 100% camo crew will bring this distance down to 300 m.
It was me, although I specifically said “high tier TD’s” since the tests I’ve seen only covered those. On Russian forum, there was a movie of the test, unfortunately I tried for like 30 min and can’t find it anymore.
All high level TD’s (JT, Ferdi, Object, ISU-152) become visible at 450 m when shooting from behind a single bush.
It’s not really opaque but like in real life there are so many variables at play… Also human nature is to remember the extreme cases (like the two you listed) and kind of forget hundreds, if not thousands, of “regular” shots.
With the A-20, your shell could have been placed on the edge of the reticle (or even outside of it, there is like 2% chance for that IIRC), which would result in it hitting (instead of the back you were aiming at) the side of the A-20 at very sharp angle, which would result in ricoshet.
With the KV, there are also a bunch of possibilities - from someone else hitting that KV a split second before you to your shell going through a viewport inside the tank and hitting ammo rack, engine and other modules there. Regular damage, plus damage from modules, plus damage from ammo rack’s explosion could result in that 30% damage.
Most of the time there is a logical explanation and with that many variables and that many statistical occurances, anything can happen. :) Plus bugs, dropped packets and lag, of course. :)