Linking people other than the primary guest to dining reservations is more of a convenience factor than anything else. You can have it not linked to anyone and still arrive at the restaurant and dine just fine. Linking someone to a dining reservation just means that when they open *their* account the dining reservation shows up in their plans.
Now, FP+ is another matter. The FP+ will "live" on the profile that has the park tickets linked to it (your MIL's copy of your FIL in your case). The problem is that *you* will not be able to make or change those FP+ because you're not friends with the copy of your FIL that has the tickets.
To correct the problem, you essentially need to delete your FIL that you created under *your* account, and become friends with the version under your MIL's account. The problem comes that you can't delete a profile that has active reservations linked to it.
So here's what I'd do:
-Go into *your* account, and re-name your FIL's profile to some other name (like Mickey Mouse), just so you don't get confused with two profiles of the same name. ALl of the dining reservations should now list Mickey Mouse as one of the guests.
-Then add the *other* version of your FIL, the one under your MIL's account. Use the "find friends through a mutual friend" feature. (To do this your MIL will have to make her own friend list visible to you)
-Your MIL will have to accept the friend request.
-At that point, go into your dining reservations and change all of dining reservations from Mickey Mouse to your FIL.
-Now that Mickey doesn't have any reservations associated with his account, you can delete the Mickey profile.
And now that you're friends with the same FIL profile that the tickets are linked to, you'll be able to make and edit his FP+ choices, if needed.
-Rob
