1. The underscore will not be required on the Special Symbols page...but that does not mean that you couldn't put it on there. We are recommending that new symbols (symbols that changed because of UEB) be included on the Special Symbols page just for clarification.
2. Yes follow print for spacing with the plus/minus signs.
3. A transcriber's note sounds like a good way to go for this.
4. Yes, follow UEB...and use the 2011 Formats rules as much as you can (without using any of the symbols that might conflict with UEB).