With all due respect, I think you need to re-read the title of this thread. This build is designed to maximize the bomb kit, and there's no better option than 30 Explosives as you benefit best having Short Fuse and Steel-Packed Powder. With Short Fuse your Bomb Kit does permanent burning and relatively strong confusion upkeep, making Modified Ammo an incredibly powerful trait on top of all the other conditions you apply. Of course, this still provides the added flexibility of taking the grenade kit if you're unwilling to learn how to survive content in melee range, but for all intents and purposes this is a bomb kit centric build and no decision in traits is decided by the grenade kit.
I don't swap to the rifle purely to stack vulnerability. I do it because it's a DPS increase. Blunderbuss and Jump Shot are both two of the highest scaling attacks we have as engineers, meaning they do more damage than the bomb kit's auto-attack. The extra vulnerability from Jump Shot is just icing on the cake.
I don't think you understand how Modified Ammo works. It's 2% per condition. Vulnerability in total provides 2% extra damage from Mod Ammo, whether it's 1 stack or 25. The same with bleeding.
You say that the trait is "inconsistent." How? A bomb kit engineer always has 3 conditions applied: bleeding, burning, and vulnerability. Even if you completely remove Glue Bomb, Smoke Bomb, and Concussion Bomb from the equation, you will gain a total of 11% additional damage putting those final 10 points into Firearms: 5% from Target The Maimed, 6% from Mod Ammo. And this is excluding any conditions the other 4 people in your party are applying.
How is that less consistent than a trait that requires you to have full endurance?
I'm not "working very hard." I'm eating Koi Cake and have 30 points in Explosives. It's a relatively small sacrifice that elevates group DPS on the whole.
I am not taking condition duration for anything but countering Defiant. It is purely to heighten the length of your vulnerability. Any added damage from burning or bleeding is just a nice touch.
Wearing Celestial/Traveler instead of Berserker/Scholar is a loss of ~300 power and 2% critical hit chance offset by a gain of 15% boon duration and 170 toughness, healing power, and vitality. You even get 1% extra critical damage.
The damage difference is noticeable, but in world events, individual damage contributions matter little. Instead, it is often healing NPCs and allies that is your main objective (i.e., Tequatl turret defense, Balthazar, Grenth). For all intents and purposes, engineers are sought after for their CC and group support, not their DPS, in such situations. That is why I recommend wearing Celestial in open world content, nevermind the added advantage of +25% movement speed. Celestial + Traveler runes is also fantastic in world vs. world, so it's an investment that pays dividends beyond just PvE.
I'm not going to continue going back and forth quoting the content of your post. Plain and simple: I disagree with your assertions, and the math doesn't really support your allegations.
Enduring Damage is in every respect worse than Target The Maimed + Mod Ammo, and none of the traits in Tools benefit the bomb kit at all. I do take Static Discharge over Invigorating Speed these days, so I should probably update the OP. But that's purely because there's not a better damage increase to take. Yes, you're getting more critical damage moving further up the Tools tree, but at a cost of less precision (meaning you're critting less) and a trait that potentially grants up to 24% additional damage---a lot of which can be done on your own part integrating skills like Fumigate, Glue Bomb, Smoke Bomb, and Concussion Bomb. At worst, 30 Tools is a marginal increase in damage. At best, Modified Ammo completely blows it out of the water.
Thank you for taking the time to write a thorough response, but I think you're viewing things at the wrong angle.