Sorry to hear about your experience on Mt. Rainier.
You already identified a few things that might have impacted the climb. By no means, I am qualified to tell you what went wrong, but here are few questions/observations –
a. Over Training: If you followed the 24-week plan, your training would have Taper weeks to make sure you are well-rested for the event. Why do you think you over-trained? Is it the volume? Intensity?
This article provides good tips and recommendations on planning (especially tapering) for an event.
b. Rest day before climb: I am assuming you had a 10-12 hour drive to get to Mt. Rainier National Park/Seattle. Did you start your climb the next day? It varies by individual, but I would have preferred to have a rest day before climbing.
c. Hydration/Nurtition: You mentioned you felt dehydrated and drained on your hike to Camp Muir. Did you hydrate and eat well before the hike? and on your hike?
d. Pace: As you said, your pace and intensity was much higher just getting to Camp Muir and this hurt your chances of summiting the next day. Not sure what you could have done differently – other than having an open conversation with the guide about how your desired pace. For future trips, you might consider doing a private guided trip or climbing with friends where you can control the pace.
Here is an article that provides some great tips on a successful expedition.
Hope this is helpful. Coaches might be able to provide better insight.
Just curious – what was the peak you climbed in the Himalaya?