For starters, what do I call this--the after-party? Second reception?
Call it Midwestern reception. "A" reception is really any gathering at which people come for the purpose of being received by the guests of honor.
Do we only invite those in Midwest City, or everyone? Something in between, like family only?
You have my thoughts above--but I also don't think there's anything wrong with inviting everyone.
Can we do inserts in the wedding invitations or does it need to be separate?
You can put it in the wedding invitations--but if you do, many people will assume that everyone received the Midwest invitation, and some people may be hurt if they don't. Or they'll talk about it, "are you going to the Midwest reception?" / "what reception?" So if you decide to not invite everyoen to the Midwest gathering, mail a separate invitation.
if you decide to invite everyone to everything, you can actually put both receptions on the wedding invitation itself.
Mr. and Mrs. James Whosit
request the honor of your presence
at the marriage of their daughter
Mr. Handsome Island-Born FiancÚ
Time and date
and afterwards at the reception
and on Date, Time
Midwest Reception Venue
Midwest Town, State
But you could also make the invitation simpler, and have just hte island events on it, and print up a second card that says,
Essay and Handsome (stick w/ first names here, I think, so avoid using your married name before it's actually yours--though if you won't be changing your name, you could use full names here)
request the pleasure of your company
at a reception to honor their marriage
Midwest Town, State
You don't need to give it any other sort of label.
And I'd put all the events on a single RSVP card, if you're using one.
How do we convey that we "expect" 0 or 1 gift, and certainly not 2?
You don't. You never, ever talk about gifts unless someone brings it up. I think most people will realize that etiquette says you give a gift for the life event (the wedding/marriage) and not for the party. They're grownups; trust that they'll give you a gift if they want to, and that if it's a hardship for them, they won't.
Not one person gave us two gifts, in our situation. I bet nobody will in yours either.
Can I still invite people we hope can join us, knowing that many won't be able to? (Some of my cousins have already insisted that I throw a party back home because they can't come it to the wedding, which doesn't even have a date yet--I'm happy to oblige, but can I still invite them to the Island ceremony? On the off chance that even 10% of my cousins can come, I would love to have them there, and would value their presence far more than a gift.)
Yes, you invite everyone that you wish would be there, for any event. The invitation says, "we would love to have you come." It's up to them to decide; you shouldn't really decide for them, even if they've flat-out told you ahead of time. They sound like they really love you, so I wouldn't worry that they'll think being invited to the Island ceremony is a gift grab. If it bothers you, I'd vote for making a "visible to family only" sort of Facebook post that says something like: "Mailing invites! Even if you told us you won't be able to come to Island, we're inviting you. Hey--hope springs eternal! And we wanted you to know you were welcome."
But truly, I don't think it's necessary. This sounds like a generous, loving bunch; they won't go looking for offense. The few people who don't get it, and will be annoyed, will complain to someone, who will then say, "What? They just want you to know you were invited."
And, if you decide to invite everyone, and all invites are together, then nobody will think it weird that they're invited to all three things.