You can clip the line off to the spool with the double ender anytime you feel the need to do it. I think it is best to wind up the spool during an ascent. You can do it with or without the double ender. Some people like to clip the spool and SMB to their BCD. I prefer to keep it unclipped and away from the body. Using the double ender to wind up the spool is a fun way to do it. If you want tension on the line just pull down harder...
Make sure to look all around you and make sure to keep the line away from your face and your tanks. The number one thing is to be still while shooting the bag. If you can do that you can do anything. Practicing this skill is essential and it is important to be able to let it go if needed.
This should never get to the point of entanglement. If I have to be cut out of my own spool while shooting a bag I should not be taking a spool underwater
That sounds interesting but generally cutting lines is a bad idea. It is better to keep it under control
In Florida cutting lines is just going to mess up somebody elses dive and would be a serious breach of etiquette. Having someone around to help out is a great idea!
Make sure to look all around you and make sure to keep the line away from your face and your tanks. The number one thing is to be still while shooting the bag. If you can do that you can do anything. Practicing this skill is essential and it is important to be able to let it go if needed.
This should never get to the point of entanglement. If I have to be cut out of my own spool while shooting a bag I should not be taking a spool underwater

the advice i got from my old LDS " have someone nearby ready to cut the line or untangle you "
That sounds interesting but generally cutting lines is a bad idea. It is better to keep it under control
