I had to write a exchange 2007 report for a client today it had a lot of handy links so I posted the link part of my report here.
That is correct you need to put the 2003 server in native mode see the following link
http://www.microsoft.com/technet/prodtechnol/exchange/e2k7help/6646fa21-69e7-49b0-9a99-839eba9e6694.mspx?mfr=true
As for not connecting to your schema master and RUS you need to run the ‘prepare____’ commands (forestprep/domainprep) before exchange 2007 will be allowed to talk to the AD infrastructure.
http://www.microsoft.com/technet/prodtechnol/exchange/e2k7help/6646fa21-69e7-49b0-9a99-839eba9e6694.mspx?mfr=true
Yes you can run Exchange12 out of your current environment and put SMTP connectors like we did with other companies. Just grow a new server put AD on it make it the Forest master /domain master, etc. then install Exchange (I gave a more user friendly link then Microsoft site with step by step if you want to attempt this on your own)
Friendly install link
http://www.msexchange.org/tutorials/Installing-Exchange-2007-Part1.html
Using Exchange12 with ADAM
From my understanding this is for performance and DR see following link – an “edge server” is a server out of your primary net… think DMZ
http://www.msexchange.org/tutorials/Introduction-Exchange-2007-Server-Roles.html
As for uninstalling Exchange 2007 I have no experience with this at all yet. My test environment is not one which I currently want to pull things out of at this moment. However Google has a few topics on the matter (none look like the answer is yes)
http://www.google.com/search?hl=en&q=uninstall+exchange+2007