I attempted to go into coexistence this last weekend but had to roll back due to not being able to connect to Exchange with Outlook 2013 externally. Everything else worked without a problem and Outlook would connect internally without a problem. OWA was able to proxy externally to the 2007 mailboxes without a problem.
I verified that the authentication setting were the same on both RPC vDirs (Exchange 20007 and 2013). I tried both Basic and NTLM for the ExternalClientAuthenticationMethod property (Basic would prompt for creds and NTLM would just not connect). IISAuthenticationMethods is set to Basic, NTLM.
Server 2008 R2 level
Exchange 2013 and 2007 are in different sites
2 Server 2008 R2 Exchange 2007 standard servers
Only one is configured with mailboxes, connectors, etc
No NLB as only one server is active and functional
IPv6 disabled via registry
2 Server 2012 Exchange 2013 Standard Servers (non R2 Windows)
Microsoft IIS ARR 3.0 on server 2012 acting as proxy for primary mail namespace (mail.domain.com)
Autodiscover is set to use mail.domain.com for resolution with corresponding scp records internally