hello Roland,
Isopropanol is a good solution for the phone. Thanks to the appropriate physical properties, it oxidizes and evaporates almost instantly. It certainly won't cause any damage to the electronics. iPhone upgrade is available in batches. Important is the region of distribution and installed internal components, major chips. The fact that one of the three has already been updated can only mean that the other two are lower on the list of system servers (maybe the production date and serial number are also important, which seems to me to be logically related to the above). Check your WiFi connection and make sure your battery has more than 30% charge. These are the necessary conditions to run the upgrade query.
Connecting the battery with the screen connected is completely normal - you can't assemble the phone in any other way. It would be dangerous to connect the screen while the battery is connected! It would be extremely irresponsible that way.