What I would suggest (follow me here - lol) - stay 3 nights at Pop with the Dining Plan, stay 3 nights at WL on Platinum, then your last night room-only at WL.
Your Platinum Plan benefits will last from check-in on the first day until midnight on check-out day. You'll have 4 credits per person per night - so you'll have 12 credits per person - and you can spread that out over 4 days. Tours and recreation work the same way, so you'll be able to do that stuff for 4 full days.
What we did when we tried the Platinum was to start our stay with 7 nights at POR on the Silver Plan, 1 night POR room-only, 3 nights Poly Plat., 1 night Pop room-only. We left Pop at 6:30 am so the last day wasn't really an issue (trust me, after all that food you won't want another full meal!!), and I think we milked the Platinum for all it was worth. I totaled up our "savings" and it was over $2000 for 3 nights. Granted I can't believe the amount spent on the actual package itself, but at least we came out "ahead" lol!
We managed to squeeze in 'Ohana, water mice, horsebackriding, parasailing, LTT, Cirque, and Fulton's the first day. PSB, Aqua Seas, Alfredo's, Fantasmic with the "preferred" seating, and Citricos the next. Third day was Backstage Safari, Rainforest Cafe, spa treatment, V&A. Last day was room-service breakfast, Dolphins in Depth, Coral Reef, Illuminations cruise, and Yachtsman Steakhouse. We did get some park time when MK was open until midnight, but aside from that we mostly ran around, stuffed ourselves, and didn't sleep. However, it was a blast and worth every penny