Hi Stephen,
ECC EHP7 is based on Netweaver 7.4 please check the oss note: 1737650 - EHP7 for SAP ERP 6.0 SP Stacks - Release & Information Note
Yes, you can install remote application server instance to use as shadow instance. SUM guide(https://websmp106.sap-ag.de/~sapidb/011000358700000783082011E/SUM10_Guides.htm) has more details about it. here is the answer to your questions:
1. Not sure how you installed, please check the message server log dev_ms and see if you see any error there. To use instance as shadow instance you can install temporary license using saplicense –temp.
2. You have to use the same instance number 81, you can shutdown shadow instance. In SUM expert mode you can give the remote shadow instance details. you have to manually start the instance in the START_SHDI_FIRST phase and manually stop it in the STOP_SHDI_LAST phase
3. If you don't have enough hardware capacity on the original upgrade system, you can install shadow instance on another host. you did correctly.
Regards
Prashanth