In order:
#1: Right steps, wrong order. Here is what I would do.
a) Get the reverse DNS record done first. It won't hurt pop3 delivery or smarthost sending so might as well do it BEFORE you make the switch.
b) Add a SenderID/SPF record to your DNS server allowiing your public IP to send mail on your domain's behalf.
c) Run the smarthost wizard. You can send mail directly and still receive mail via pop3. But sending mail directly will work best if you know your reverse DNS record is set and you've set up a SenderID record. Monitor logs and queues, send some test mails, make sure this is working.
d) Run the IAMW and configure your server to receive mail directly. It can be *configured* to receive mail directly and still use the pop3 connector at the same time. Test your server vy trying to simulate an email being sent directly. Since your MX recods will still have mail getting delivered to your hosting company, you'll have to use telnet to test. Plenty of tutorials on how to do this. USe a machine outside of your network for testing so you know your firewall works as expected, exchange response as expected, and you can get a real message to your server as expected. There are also some web based tools that can test a specific IP (mxtoolbox.com had one last I checked) but regardless, KNOW your exchagne server can receive email before flipping the switch.
e) Now make that last DNS change, update the A and MX records to send mail directly to exchange..
#2: If you've purchased a certificate for remote.* then you should make sure your DNS changes and the address used in the IAMW above reflect that. Otherwise you'll have certificatre errors. In short, don't use mail.*
#3: Since DNS takes time to propogate, leaving the pop3 connectors running for a few days is not a bad idea. When you are satisfied that all mail is now getting delivered via SMTP, simply use the pop3 wizard on the SBS console to remove all pop3 accounts.
#4: You'll discover if this is the case when you perform step c above. Hence the reason for testing and doing each step completely separately and not in one big bang. If your ISP connection is "business class" then you can probably make the switch without any intervention. Residential class connections may filter port 25, in which case you'll want to work with your ISP to get the appropriate ports unblocked and make sure you arent breaking any terms of service by standing up a mail server on your connection.
#5: Not for the type of switch you are doing. You'd follow those steps if you had a specific need to allow home uers access to their work mail...and even then I'd argue that there are better ways. But if that time comes, it'd be a separate EE question at that point. In short, not needed for converting from smarthost/pop3 to direct SMTP ingoing and outgoing.
