We will be at MK next May, the week before Memorial Day. I am planning to use my first FP of the day for the 7DMT. However, if we get there at rope drop and go straight to the ride, will we need to use a FP? Also, if I do book the FP for 7DMT and someone in my party wimps out, will that mess up anything for my group's 2nd and 3rd FPs?
Guests never NEED to use a FP for any ride. You can always use the standby line. Guests always have that choice. (bear with me, I'm not trying to be rude.)
Second, none of us can say exactly what the standby wait will be when you arrive at the queue. I've been to WDW for many a major holiday, and many factors are at play. Bad weather = shorter lines. Also, lots of time we TRY to hit RD, but don't quite make it for a host of reasons.
It is entirely a personal choice to decide how long of a wait = an acceptable wait. Every day at least one group is first to ride. None of us can say who quickly the wait will go from 0 to one that is unacceptable to your family.
If you book 7D - for ANY member of your party- and they don't end up using their FP - for any reason- then you are able to rebook it for something else, SO LONG AS- there are still FP available for other attractions.
You cannot book your 4th FP, until you personally have used your first 3FP. that is the impact. Your #2 and #3 FP are safe to use.
From here, none of us can say what kind of FP will be available to your family if they don't use the 7D FP. This is important! But if they don't use it, they are entitled to re-book their FP entitlement and use it on a different ride.
What we can say is the earlier (in the day) you rebook that FP, the better. If part of your party uses a 9am 7D FP, and part of your group doesn't- then you have two options. Option A would be to swap bands and let the willing folks use the FP. (then everyone is equally able to get their 4th FP at the same time.) Option B, is for the non riders to get an alternate FP and USE it before everyone has used up their first 3 FP. That might be simple or it could be a pain. Option C is that the ones who don't use their first 3 FP opt out of getting their 4th FP. (probably not the best option). Option D, everyone enters the queue, scans their bands - but exits AFTER the second scan. (FP are scanned twice, and considered used once a guest passes the 2nd scan.)
I will also say, 7D is truly a kiddie coaster. If part of your group doesn't use their 7D FP, they might find limited options for picking a different attraction. Then again, anyone opting out of 7D probably doesn't want Splash Mountain anyhow.