Minimum iOS SDK Version for iOS 15

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.

The latest iOS Development tools (as of mid-September 2012) no longer support iOS devices running a version below iOS 4.3. So apps compatible with devices that can't run iOS 4.3 drop off a cliff as developers update their apps using the current Xcode, which use is required to fully support the iPhone 5.

Some developers support only down to iOS 5.1, as that's the highest supported OS for the 1st gen iPad.

Warning: If you have apps for such older iOS devices on iTunes on your Mac, don't update the apps unless the required minimum OS is appropriate. Or make backups of your iTunes library before you update. iTunes will throw working versions of apps in the trash after downloading a recent update, and may even delete these apps off of your older device (which is very bad).