IOS – use the enterprise developer account to sign Passbooks that will be used externally

Each client will need to have an enterprise account. If they want to, they can share their distribution key with you, and you can build an app for distribution within their organization. If the number of clients isn't large, this shouldn't be a big deal -- you can just set up a different target for each client, and then use an aggregate target to build them all at the same time.

Over the air distribution is definitely possible. One way is by making the app available for download on a restricted access web server; another is to install it directly on any managed devices using mobile device management.

I believe most of your questions are answered in the enterprise program agreement. Give that a careful reading if you haven't already. Questions about how to interpret a legal agreement are probably off topic for this forum, but I think you'll find the agreement to be fairly easy to understand as legal documents go.

Update: Another option that's been available for a while now (though possibly not when I wrote the answer above) is custom B2B distribution. You can offer your apps for sale through Apple's Volume Purchase Program directly to your clients. That is, your clients would purchase through the Volume Purchase Program (essentially the App Store, but multiple licenses) and then distribute your app via Mobile Device Management or redemption codes. The down side of this plan is that Apple would take the usual App Store cut of the selling price.

I suppose I should begin with the caveats that

• I do not live in Europe so I do not know how things might work there.
• I do not own or manage a business. So also clueless there.
• I have not personally ever bought anything other than free apps from the Apple App Store.

So, what I am saying is that I can not answer your question. But I also suffer from Y-chromosome induced Male Answer Syndrome, so I'll try to answer anyway.

Let's start with the assumption that Apple wants to sell you these apps. You just have to track down someone to help you either at Apple or at the company which sells this Afaria app you mentioned.

You do not mention which country in Europe you are in. But I assume there is an Apple web site either for that country or for your native language. I suggest you go to that web site and search there for information on a volume licensing program.

For example, I tried looking at http://www.apple.com/uk/ and searched for volume license.

That search turned up this link: http://www.apple.com/uk/mac/volume-licensing/

Now the page that link took me too appears to apply to volume licensing only for the OS X Lion operating system. Still, the people involved would at least know something about Apple's Volume License process. So, if you were in the UK I would suggest you contact them and start a conversation.

Even if they can't help you they could still probably point you towards other Apple people who might be able to help you.

I suggest you try something like that with the Apple web site which targets your region in Europe.

I realize this would be a tedious and frustrating process. And I hope someone else provides a better answer to your question here. But unless they do, I think your best chance of solving your problem is to start working you way through people at Apple who would want to sell you something help you.