Sounds like you’ve done everything correctly as far as I can tell.
I think to use the codesign command, you will need to rename your .ipa file as a .zip and then extract the files. You should have a folder called Payload, and inside that a file ending in .app. Point codesign to the .app. Will be interesting to see what version it says. (actualy, on Windows the .app is a folder can can be opened, but you shouldn’t need to do that).
Out of interest, are you remote building using a mac, or just doing a blueprint only project that wouldn’t normaly require a mac? It’s interesting that you are getting the iOS SDK version error.
I assume your xcode is up to date. I don’t use it either although I am remote building which I guess is using it behind the scenes.