And after re-reading your OP...
You do know that the resort buses do NOT arrive and depart from TTC.
That ended years ago.
The resort buses boarding area is near the MK front gates (on the MK side of the lake).
That's why I said to take the resort monorail to MK and walk over to the buses.
If MK closes at 8pm, the buses will still be leaving up to 9:30pm or so.
The Electrical Water Pageant starts at 9pm at Poly beach and runs for about 7-8 minutes.
It will take you 10 minutes (approx. its a long way) to walk to the Poly monorail station from the beach. (And its still closer than walking to the TTC station.)
Another 10 minutes (approx.) to ride to MK (monorail makes a short stop at Grand Floridian station).
Another 5-10 minutes (approx.) to walk from monorail to the MK bus loading area.
That puts you AT the buses at about 9:30 - 9:45pm... which is "pushing it" with an 8pm MK closing time... you might have just missed the "last bus" to POP by then.
ALTERNATE ROUTE:
(Since Epcot "closes" at 9pm, Buses run until past 10pm)
Poly Beach until about 9:10pm
Walk to TTC directly from beach (12-15 min.)
Monorail to Epcot (12-15 min.)
Walk to Epcot buses (10 min.)
Bus to POP (approx. 9:50pm)
The "Pageant" is cute, but not a "biggie" (especially after Wishes), so if you don't want to spring for a taxi, I'd not wait that long to see it.
(I hate that late night trip to DTD/PI and changing buses to your resort).
If you don't mind a SPOILER, here's a link to viewing a video of the Electrical Water Pageant.
Video of the whole show.
(SPOILER ALERT):
http://video.google.com/videoplay?docid=-7715818496829823091