Just a question, have you tried convert the entity files from the last sins of a solar empire v1.195 (the last version). For Diplomacy the last is the v1.37 and for Entrenchment is the v1.055.
Perhaps some people find crazy edit the older SOASE but at difference from Rebellion, it works when you are out the net without steam. Yes steam can work disconnected but first you need connect one time and after a few time, probably it will not work more, specially if you install any program and probably when windows makes their own backups from the register.
At the end, I can edit the old game working with some stuff for the newer Rebellion and enjoy it.