Let's solve the bowel issue (for everyone's sake!), esp before WS.  Did you drink enough water the day(s) before?  What did you eat that had your bowels "stocked up" but yet you were starving?  
Ideally you're eating more carbs (percentage-wise) the couple days before; drinking plenty of water; and -gaining- a few pounds of weight (the water that's stored along with glycogen), and also helpful to avoid constipation.

How do you use caffeine before/during the race, any unusual differences there?  How much did you go the previous couple days, and how much in total did you go during the race?  (Not sure if everyone wants the literal answers to those questions, but trying to think about solving the problem...)

I don't think it's the taper itself.  While a normal daily run is helpful for many of us as part of our "routine," the single 0M-day shouldn't have completely thrown everything off.  And I think you run different paces at different times of day, so you're body isn't regularly dependent on the same run at the same time of day like some people are. Sounds like food/drink during the week is affecting it, and maybe longer.  Do you eat your fiber?  Something like oatmeal and fruit (plus yogurt for protein) for breakfast, fairly regularly, might help clear out any malingering collateral damage from previous Burger Wars. 
Last is the possibility of a food intolerance or allergy, which would take some time to narrow down, but you seem to be fine normal training days, right? 

And: don't take off at the front of the pack! =)

Still a solid training day.  Mostly, you'll probably feel better during WS just from eating better.  

I didn't drink any more or less water the day before than normal.

We had pizza the night before.  Family in town.  Everyone went out.  I went with it.  I have a gluten issue so pizza is not often consumed.  Although I do just fine at this place we went.

I didn't go aggressive on caffeine for this race like I do others.  Just hit coke at all the aid stations.  No other sources except maybe a gel block flavor had it.

Food will be my crutch through this whole thing I think.  Just shows I don't have it nipped yet.

We'll be up in Pbville May 25-27 and June 1-3 if you're around and want a run!

I will be there...probably.  You know where to find me.

I thought my Boston crap record would stand forever, you smashed it by the 18 mile mark.  A Beamon-esque performance in that area.

Do you remember what you did in terms of food before Boston? My advice would be that if you know what you did and it worked stick with it. I try to eat the same things leading up to a race because I know that they will work and not cause any issues. It is a little boring to always eat the same thing but eliminates one variable that could cause problems. 

Your plan is good: work out the steps before the big dance.

Long descents 2x per week for the next 4 weeks.  Go.