Winning Race for the Galaxy Without 6-cost Developments?
I don't want to make this sound too much like an argument from authority, but I've won the WBC RftG tournament 3 years in a row.  I also mantain the Race for the Galaxy Statistics page, which has some interesting play data on cards from 160,000 games played.

(1) 6 devs are usually important for winning, but not required for doing so.  I build at least one 6 dev in more than 80% of the games that I play.  


On the other hand, calling explore +5 early game isn't really a good move unless your whole hand is bad.  It's better to establish card flow so that you can keep most of the (somewhat fewer) cards that you see, instead of repeatedly exploring, which will leave you poor.  Repeatedly calling explore +5s leaves you too far on the quality over quantity spectrum.  It's not very useful to have great cards that you can't afford to build.  6 devs, especially multiple 6 devs, aren't great to have very early in the game, because they soak up valuable hand space that you need to get your card engine going.


(2) Produce/consume is indeed the most viable way to win the game without building a 6 dev.

First, basic strategy.  Early game you want to get card flow.  The best way to do this is to settle a windfall world and trade off it.  The second best way is to settle a production world, produce on it, and then trade.  Windfall settle/trade is better than Production world settle/produce/trade because it is a turn faster.  This is pretty general opening advice for any strategy.

(3) The key to a produce/consume strategy is to build production worlds early.  So ideally you want to settle/trade a windfall, and then call some mix of settle, trade, and produce.  You want to avoid calling develop, which mostly fills up your tableau without increasing your production capacity, until you have at least 2 production worlds and a windfall.  After this, you might want to dev once to put down an efficient consumer card.   Mining Conglomerate, Diversified Economy and Consumer Markets are the best non-6 cost devs for consumption in the base game.  Free Trade Association and Mining League are good 6-cost consumers.  But you can likely use an opponent's develop, or settle worlds with consumption powers instead.  Galactic Trendsetters, Tourist World, and Old Earth are the best worlds for consumption in the base game.  You want to call trade rather than consume 2xVP until you can get at least 6 VP per consume 2xVP.  Also important with a consume/produce engine is the ability to draw cards so that your engine doesn't stall as you are produce/consuming.  It's especially good to have cards that draw on produce.  This makes cards like Lost Species Ark World, Mining World, Comet Zone, and Gem World particularly good production worlds in the base game, as you can see at the graph on the base game rftgstats page.

Addendum: How does this change for expansions?

Produce/consume itself becomes marginally weaker in the expansions.  With the card stats by game version animated graph, you can can see Galactic Trendsetters hastily moving left (less often played) and down (less winning when played), which is a bread and butter produce/consume world.  This is due to two factors.  


The addition of goals, which give an extra point source that is mostly rewarded for building.  P/C itself sometimes misses early builds to set up production capability.  
The second is the introduction of the card Improved Logistics, which lets players settle twice in a single settle phase.  This card lets opponents quickly end the game, before a produce/consume player can get many cycles in.  


However, the card Alien Toy Shop by itself is a 4 VP/cycle engine, which helps P/C a bit.

Still, the fundamentals for produce/consume are the same, but the fraction of times you want to produce/consume should decrease with the introduction of the first expansion.

The expansions also introduce another viable way to win without building a 6 dev, the tableau rush.  Since you can now get a substantial amount of points from goals, especially with the aid of improved logistics, you can grab a couple of goals and end the game really quickly with a middling score in the 30s.  But remember, the game is not about scoring big, it's about scoring more than your opponents.