At a high level, do you have any teachnical reasons as to why you do not want to go through the App Store process and use the Custom Apps option?
Our organization has an Enterprise account along with an App Store account and have started the migration to App Store deployments as they provide a number of operational benefits that Enterprise applications do not. I would encourage looking into this as in the long run this will enable you to be so much more efficient along with provide a better customer experience. Apple has already invested a great amount of time to understand customer satisfaction, so if you can leverage that, it would be a win for you unless you have a team that can do it better than Apple for your specific organizational needs.
As for your Apple accounts, you should be able to use the same DUNS number (process could have changed since the time we created ours), however, as of now, an Apple account can only be the agent on a single Developer account so you will need to have multiple Apple accounts as Agents.
The only technical limitation that you are going to run into with Sharepoint is that as you have probably seen on the web is that your URLs need to be publically accessable to Apple so bascially the entire web. Apple will need to verify the installation point and IPA file to allow the device to install the file. The way that you secure this is up to you, but you would want to check with your Sharepoint administrators if they are going to be able to enable you to do this with your organizations policies. Outside of that IPA delivery is just like downloading any other file on the web.
You will not run into an issue if your company domain is "companyforsale.com" and the domain you host on is either a sub domain or a completely different domain. I would still recommend using the Custom Apps approach as I think that it will serve you better in the long run if you take the time to invest in learning how to use the system to your advantage espectially if Apple decides to completely end the Enterprise program in the future.
Hopefully this helps!
One of the main reasons for thinking about the Enterprise account is to avoid the Apple review process associated with the app. This isn't for any sinister reasons, it's just that our apps will be in a format and contain content which I know from previous experience will be rejected by Apple if it is intended for the AppStore.
They are perfect for a sales force and do not have to be publically available.
I imagined that we'd provide a URL to our sales people hosted on Sharepoint (or similar) and they'd install the app on their devices after the IPA file has been approved by the users. The URL's to the apps would be behind a password protected area - both because they are not to be publically available and also because I understood that Apple wouldn't allow apps developed via the Enterprise program to be accessible by 'the public' and on the web?
Thanks for your help.
1. Yes, holding both accounts are possible, and, I believe, not unusual. Just treat them as separate and use two different/unique Apple IDs, not one for both.
2. As for 'closing' the dev account, there is no provision for that, other than simply to stop using it until it expires.
3 - see chuck's reply