Hi David,
I will answer the questions one by one.
Q1: Does each distinct email address count towards the limited number of Office 365 users?
A1: There are 10 licenses available in Office 365 for small business preview. That means you can create 10 users who can use the services in cloud. If you create more than 10 users, others will not be able to use the services in cloud.
Q2: In researching the previous point I saw a comment that Small Business Premium cannot bring in subdomains. If that is true, what are our options?
A2: Yes, we cannot add a subdomain in Office 365 for small business currently. For the detailed information, please refer to the link below :
http://technet.microsoft.com/en-us/library/jj819265.aspx
If you have some concerns about the "subdomain in Office 365 for small business" feature in Office 365, I would suggest you submitting a feedback at http://support.microsoft.com/common/survey.aspx?scid=sw;en-us;2222&altStyle=MFE&renderOption=OverrideDefault&showpage=1&fr=1&nofrbrand=1
Your useful ideas and feedback will help us improve our products and service. Your time and efforts are highly appreciated.
Q3: Would we be better to leave our existing site outside Office 365 altogether, creating a new office site in Office 365, to which we forward the handful of office addresses? If we do that, how do we get our outward "Sent from" addresses correct?
A3: For this question, I want to confirm your requirements:
If you want the website hosts on previous server, please refer to the links below:
1. P plan and not change name server
If you use Office 365 subscription Plan P and you do not change the name server records for your domain, please refer to the link below and just add the required DNS records for your domain to work with Office 365 services without redirecting name server records:
http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/hh416759.aspx
2. P Plan and change the name server
If you use Office 365 subscription Plan P and you have changed the name server records to point to Office 365, please refer to the link below for how to move your email to Office 365 and keep your current public-facing website:
http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/hh335299.aspx
http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/hh335298.aspx
3. E Plan
If you use Office 365 subscription Plan E, please make sure you do not select SharePoint Online service under Domain intent when you first add and verify your custom domain in Office 365.
Please check it by following the steps below:
1). Log in Office 365, click Admin
2). Click Domains->Domain properties
Make sure the SharePoint Online is not selected.
If you use a same domain between cloud and on-premise, you want to configure the mail flow between cloud to on-premise, please refer to the link below :
Simple Domain Sharing for SMTP Email Addresses:
http://community.office365.com/en-us/wikis/exchange/simple-domain-sharing-for-smtp-email-addresses.aspx
If I have misunderstood the question, please provide the detailed information about the question for us to search.
Thanks,
Linda Wang