You absolutely should not feel guilty! Your bridesmaids are there to support you in the planning process. I would only recommend nix talking bachelorette planning with people not attending the event, but it sounds like this bridesmaid is still intending to come with, so it's not like you can cut her out of the planning.
If it helps you work through this, I had 9 bridesmaids but only 5 came with to my WDW bach, and only two of them are as big of Disney fans as me. I primarily relied on my co-MOHs for major planning decisions, and would weigh major cost-related decisions (travel costs, rental car, resort) with the 5 who were coming.
I think you're contributing SO MUCH to this already that you don't need to worry about backpacks. Instead, and it already sounds like it, focus on this to put in the backpack or coordinated accessories for everyone to wear! Again, don't feel guilty for not getting a bag that they probably won't use again, and if some of them want loungeflys they can get their own.
This sounds like so much fun!!! What a great plan. Just my .02 but I would split this into "required" and "suggested" activities. We basically ended up all staying together and doing rides together (with some opting out of the "big kid rides" and just taking a break or shopping), but I really only intended everyone to be together for meals, drinking around the world, the fireworks cruise, and our photo session, with lots of "free time" for everyone to do what they wanted. We went in 2021 before G+ was released, but you may want to ask if folks are okay with adding in a ILL for Tron, Guardians, FOP to your schedules. And, 1-2 of my bridesmaids would go back to the resort for a break each day and would reconvene later, so briefing everyone on the WDW bus system is super handy!
Finally, have you looked into DVC rentals for Saratoga Springs or Old Key West? We looked into it, but needed to find a seller with cheap, distressed points in order to make it work for us.
Just to end on how much fun this trip looks and I commend you for putting so much effort into this!! It will be such a great trip!!