These questions are general questions that are not specific to Sprint deployment.
1. You should always use SIP provider's network for SIP trunk delivery which will make the connection a lot more secure, and for some large providers there is no other option, which sounds like the case here
2. It may not be necessary to use CUBE if you do not require SIP trunk authentication as you can create the trunk directly from CUCM, but it is highly recommended to deploy CUBE for may reasons, such as demarcation, internal address hiding, security, sip header manipulation, early media negotiations without needing MTPs, etc, etc
HTH, please rate all useful posts!