I haven't taken the time to read through this whole thread...So some of what I say may have been touched on already.

First, you say 10 minutes before total dark. That to me sounds like you are past legal shooting hours.
Second, if you couldn't see where the arrow hit on the deer then you took too far of a shot when you didn't have enough light.
Third, 35 yards with a mech head, and you still wonder why you didn't get good penetration? When using mech heads haveing a precisionly tuned bow is even more important because the head takes up so much energy on impact so it can open. Any little porpoising of the arrow will increase that energy take up by the broadhead exponentially.
Fourth, dude, your a rook bowhunter taking a 35 yard shot? Heck most pro's don't take shots over 30 yards at whitetails...and that's why they're pros. They know deer better. What is it, sound travels at 700 some odd feet per second. If your arrow is traveling 280fps, that means that deer that is 105 feet away, can hear the sound of your bow after .15 seconds, when it takes your arrow .375 seconds to reach the deer...and that doesn't even account for arrow speed loss due to drag. That deer has .225 seconds to react to the sound which in this time of the hunting season it will react without question. Gravity affects objects at 9.8 meters per second. That a rough equivalent to 32.144 feet per second. Divide that by ten and you get 3.21ft for every tenth of a second. So that deer had the opportunity to drop 6.42 feet from the time it hears your bow til the time the arrow makes it to the deer. I feel I was being pretty generous with the arrow speed as well.

I'm gonna have to say that the deer dropped and spun back away from it's traveling direction. Yet, you found blood halfway up your arrow which is more than enough penetration to nail both lungs. Since you failed to find the deer chances are you gut shot it and it had a nice long painful death...