I noticed a lot of these things on my first playthrough, long before I knew anything about the conspiracy theory videos. I mentioned these to my roommate, who was actually a QA tech for ME3, and his best guess was that it was simply a resource saving trick, recycling some existing imagery that tied in visually to the rest of the game. The colors, the shrubs, Anderson and others talking about how nobody made it to the beam... to me it all suggests that Shepard is bleeding out on the ground and is only imagining everything else that happens. When you get the ending with battered corpse in N7 gear taking a breath, it looks more like the rubble in London than the remains of the Citadel, so again this seems to support the dream sequence interpretation.
As for the other choices throughout the series mattering/not mattering, I think that the biggest issue here is that many of these arcs are resolved prior to the big grand finale battle for Earth. The Geth/Quarian conflict is ended one way or another, and the Turian/Salarian/Krogan issue even earlier than that. I was a little disappointed that the Asari's fate is so linear, Javik's revelation that the Asari were groomed to be the best hope against the Reapers came to nothing, and that the Asari getting boned is just a backdrop for the Kai Leng/gunship boss fight. Eldargal's suggestion about choosing between Palaven and Thessia would have been incredibly emotional. Garrus is a total boss, but let's face it damn near everyone picks Liara for their romance option.