For those who were wondering, I came away from my single acupuncture session not feeling much differently than I had when I went in. Truth be told, more might have happened if I'd gone in for more than one session—I wouldn't rule that out after having been only once. At the very least, it was an interesting experience and I can now check it off the "I might like to try that out sometime" list.
I did go in to see my doctor on Friday, two days before the ride. She checked me out and diagnosed me with Achilles tendinitis, which is essentially what I thought I had all along. She told me I hadn't done any major damage, everything was intact, and I should gauge my riding ability on Sunday morning by how I felt; readjusting the position of my pedal cleats might also help with my range of motion, so I figured I'd give that a shot.
I did, and my ankle felt "okay"—workable, tolerable, bearable—during the three or four warmup laps I did around the parking lot prior to the ride start. So I decided to try riding 70 miles. I survived—72 and a half miles, to be exact; maybe a little longer, since my cycle computer didn't kick in right away— through pouring rain and wind smacking raindrops and road grit in my face. And I had a blast. By the end, the ankle was sore, and it's aching a bit again now, though less than last Saturday when all of this began.
Now, hopefully, I can be a good patient and give my ankle the ten to 14 days of rest it requires and deserves.
Thanks to all for your suggestions and input!