Regarding rear shot pilot kills
Yes the cockpit is now better protected, meaning that there are hitboxes better protecting it from rear-angle shots. It is less exposed than before and requires a shot from 4 'o'clock or 8 'o'clock to have a chance of being hit. This should make it less common to get rear shot pilot kills. However I guess that micro lag can sometimes cause the bullet (which travels almost 2000 ft/sec) to sometimes skip a few hitboxes and not register it's position until inside the pilot hitbox, causing a ghost shot pilot kill. This would then need a server rehaul to be fixed. I can't be sure this is the issue but i suspect it.Regarding B-17 hitpoints
I guess you were flying the P-39 with the 37mm M-4 cannon? Do you have a link to the video? The thing of interest is to see exactly where did each of those 5x 37 mm shells hit. Remember there is no partial damage in Warbirds, so if they hit different areas of the B-17 then you would now have several parts with low hitpoints, but still no part that is 100% destroyed. The B-17 is the largest and toughest bomber in the game, and there were several ww2 records of B-17's with half of the aft fuselage shot off but still got home. Never shoot at the aft fuselage of the B-17, but go for the engines or a front/side/top shot on the cockpit. While we are missing the graphics to show large holes in the aft fuselage from damage, the B-17 would still be able to fly until the tail is completely destroyed. Note that the B-17 has not been given extra hitpoints to be extra tough. The hitpoints have been set using the same formula, based on size, as for other bombers. It's just more massive than the rest.
You can use the picture below to assess how many shots you need to kill the buff, depending on where each bullet hit. As a general advice the hitpoints of the B-17 engines is less than 1/10 of the aft fuselage hitpoints, so always go for the engines. It will save you time and ammo. Wether or not the hitpoints vs gun damage is fully representative of real world results I don't know, but when remaking the buff hitpoints I pretty much kept them at the same level as they were before, as to not change the balance of the game. They might probably be a bit tougher than in real life, which is to compensate for the fact that you can't fly in 100x bombers formations in Warbirds and lone buffs are more vulnerable than a tight large formation. The difference is that while all buffs previously had a generic hitpoint pool, regardless of relative size, the buffs now have the toughness of every part set depending on the hitbox volume, and the engines can no longer take over 10 direct 20mm rounds without being damaged as before.
Below are hitpoint values for some parts of the B-24 Liberators. Go for the engines, not the fuselage.