60% Sour Onion Rye (help/suggestions)

Hello, 

I'm at my newest creation this past week with a couple of attempts down. (sorry no pics) I am using toasted dehydrated onions stone ground rye and high protein flour.  I am building a rye starter with my white levain.  

My times have been such.  

Build Rye sponge (16-18 hours) at 100% hydration  

Mix on low until dough comes together and develop slightly. ddt 80 

(Not sure if I'm over/underdoing it here)

Bulk ferment 3 hours at room temp with 4 - 30 minute

s + f's

shape into a batard

proof in warm area for 2 hours.  bake with steam for 15 and 

convection 25-30 minutes longer .

The total fermented flour is 22% (rye)

The total weight of the levain is 25% of the total dough weight.  It is just shy of 50% bakers percentage.  

My first attempt was at 85% hydration and the loaves were wonderful in flavor had a decent crumb but cracked and flattened a touch.  I did a follow up lower the hydration greatly just to see.  At 75% these were heavy/dense but held there shape much better.  Still both had little oven spring.  

My goal for structure is something like the 66% sour rye posted on the forum I believe made by khalid.  

http://www.thefreshloaf.com/keyword/sourdoughryebread

So a few questions.

1) Is my sponge too small?  This was a simple change I thought to incorporate after looking at many recipes.

2) Are stretch and folds helpful with high % ryes or are they better off without?

3) Many of these recipes call for the addition of yeast.  Is this the trick to some extra oven spring and overall volume?

Any suggestions/comments are welcome

Happy Baking

Josh