1. Creating an Intrado route pattern alone in CER does not impact anything.
2. Check out link below for Conventional ERL to Intrado ERL migration
https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cer/10_0_1/english/administration/guide/CER0_BK_CA66317A_00_cisco-emergency-responder-administration-10_0/CER0_BK_CA66317A_00_cisco-emergency-responder-administration-10_0_chapter_0101.pdf
3. You do not need a separate CSS to test Redsky in your scenario. Here is what you need to do.
In CER, create an Intrado Route Pattern, let's say 1111.911. Next, create an Intrado ERL and select 1111.911 for route pattern under the drop down list then enter your ELIN in the ELIN field. Next, I suggest you assign a test phone or a few test phones manually to this ERL for testing. You could also use the switchport method but do not use the subnet method as that could affect production phones. After that, create a route pattern in your CUCM that matches the intrado route pattern (in our example, it would be 1111.911) and point it to your Redsky SIP Trunk. Before you place any test calls, make sure you ask Redsky to place your setup in test mode on their side.
Finally, you can dial 911 from your test phone. The dialed digits will match the same 911 pattern as any other phone in your cluster and route the call to CER. In CER, since this phone is manually configured under an Intrado ERL, the called number get xlated to 1111.911 and the calling number gets xlated to the ELIN you configured for that ERL. The call goes back to CUCM and match the Redsky Route Pattern 1111.911. The call gets routed to Redsky test environment. Once the call is connected, you should hear an automated recording announcing your calling number and the address associated with that number.
