Reinhart Yeasted Rye was gummy -- why?

Reinhart's Yeasted Rye was gummy

This weekend I decided to try Reinhart's Yeasted Rye, from Crust & Crumb p.104. I realize sourdough has its advantages when making rye, but the reason I went with the yeasted is that, sadly, my wife doesn't like even the mildest sourness of sourdough. I followed my notes (below in italics) based on that recipe closely. I had to use half high gluten, half medium gluten flour for the bread flour, and used half pumpernickel, half whole rye for the coarse rye. I also brushed the top with an egg white and milk mix, then sprinkled on colorful seeds.

Reinhart's Yeasted Rye: Combine sponge ingredients and ferment, covered, 4 hours, to rise & fall: 126g bread flour, 126g coarse rye flour, 1 tsp instant yeast, 1  c cool water;  After 4 hours, add to sponge: 252g bread flour, 1 TBSP brown sugar or diastatic malt powder (I used the latter), 1.25 tsp salt, ½ tsp IY, 1 TBSP caraway or other seeds, 1 tsp caramel coloring, instant coffee or cocoa powder, optional (I used 1/2 tsp cocoa powder), ¼ c buttermilk (4 TBSP). Mix on low in machine 1 min, then medium a scant 8 minutes; don't overmix. Should be smooth, stretchy, tacky, evenly incorporated. Should windowpane. Ferment covered until double. Shape (e.g. for bread pan), put in greased loaf pan, cover, proof until nearly double; bake in preheated 350°F (177°C) oven for pan bread, with steam, 45-55 min. until internal 195F. Cool on rack 90 mins.

Ok, on my first try I lowered the oven temp slightly (about 170C) in an attempt to get a softer bread, baked 55 mins, and tested to 195F internal so I figured it must be done. I brushed the top with milk to soften the crust, as it was to be sandwich bread. I noticed a slight collapsing of the top crust at that point. I let it cool in the loaf pan 15 mins then tried to turn it out onto a cooling rack, but it started sagging, collapsing slightly.  After 90 mins cooling, I cut a slice and found it was all gummy inside (inedibly so), despite having been very brown on the outside and having reached the requisite internal temp. I doubt I overmixed, as Reinhart calls for a scant 8 mins mixing, and I probably mixed 6 mins, stopping since it seemed smooth and springy and didn't tear when trying to get a windowpane of sorts.  So was this more likely to have been underdone, or to have been that starch attack thing?

I then made a 2nd (successful) attempt, but being more interested in getting a usable sandwich loaf for today's lunch than in doing a proper experiment where only one variable at a time is changed, I moved the rye flour from the sponge to the 2nd stage (and an equiv. amount of bread flour from 2nd stage to sponge), switched from diastatic malt powder to brown sugar, scalded the buttermilk, kept the temp at 177C (i/o 170C), tenting with foil after 15 mins, and baked to 203F internal  instead of 195 internal. I let it cool in the loaf pan half an hour before transferring it to the rack, and there was no sagging or collapsing. After 90 mins cooling, I sliced a piece. The result was a perfect loaf with a perfect crumb, nary a hint of gumminess, and a lovely, light caraway rye flavor.

Now, other than telling me to do sourdough (I understand that the higher acidity of sourdough will inhibit amylase activity in the sub-176F range), what advice do the rye bread gurus have for me? Which of the above bolded changes do you think was most likely responsible for the drastic improvement of the 2nd loaf?

Do you think it was the lower baking temp combined with a non-acidic dough giving more time in the starch-attack temp range which had caused the serious gumminess in loaf #1? Would the presence of diastase have exacerbated this? Would enzymes from unscalded homemade buttermilk exacerbate this? Or is is possible that the loaf was just underdone despite clearly reaching the specified internal temp and being nice and brown on the outside (well, the top)?

I've already succeeded in changing the recipe to produce a successful loaf, but I'd like your input on which change(s) probably helped the most. Thanks in advance! :)