Sourdough muffins a la Reinhart

So...I've been playing with Ken Forkish's 100% hydration starter from Elements of Pizza, and I've got one that works fairly consistently.

On Friday, I refreshed the starter (discard all but 25 g, add 100 g of 85 degree water and 100 g of flour). I let it sit out on the counter until it had doubled, then stashed it in the refrigerator.

On Sunday, I refreshed it again, but this time used the discard (200g) to make Reinhart's english muffins from The Bread Baker's Apprentice.

First, it works out that that 200g is roughly a third of the 10 oz of flour called for, and I decreased the milk to account for the liquid from the discard. (I've heard that taking 1/3 of the flour content is sufficient to convert a conventionally yeasted bread to sourdough, when also adjusting hydration to account for the water in the starter.) 

The muffins turned out good, with sufficient nooks and crannies. Problem was that they took about twice as long to rise as the book suggests for instant yeast. Bulk fermentation took about six hours, and after shaping, thy rose about three.

My questions are these:

1. Should I just expect a longer rise time from sourdough breads, and if so, is double the proofing time a good rule of thumb? Or is my starter not active enough? Which leads me to:

2. Should I be stashing the starter in the refrigerator before it has a chance to double?

3. Does starter become levain at some point, or are the two terms used interchangeably? Let's say I'd taken my starter out of the fridge, fed it, and returned the original starter (diminished by 25 g) to the refrigerator. So now I have a new starter that I intend to use to make bread (the one I just fed). It's identical to the starter in the refrigerator, except that it weighs 25 g more. By convention is the new starter (assuming it all goes into the bread) now called a levain?

4. Finally, when should I add this levin-starter and to a bread I'm making? Just as it gets bubbly? After it has risen a bit? Or should I wait for it to double at room temp? Or should I stash it in the refrigerator (after it doubles?) and make bread the next day?

5. One more: will the amount of time the levain sits out and rises affect the acidity and sourness of the final dough, or is that strictly a product of the culture itself? Those muffins, when eaten alone with butter, were fairly sour, despite the fact that the starter alone smells very like an english muffin.

By the way, Elements of Pizza is a phenomenal book. I've made pizzas I didn't think were possible in a home oven.

Thanks for any thoughts.....