If you get a positive pre-read from Admissions, that means all the information they have reviewed qualifies you for acceptance with coach support. What could go wrong? Well don’t submit a rude or ungrammatical essay (but you wouldn’t do that anyway). And of course your recommendations will be in order. Also be sure to stay in school and out of jail!
After the coach confirms that you received a positive pre-read, and after s/he confirms that s/he has allocated one of the team’s slots to you, what does that mean? Each coach has a certain number of applications s/he may support through the admissions process and it means your application is one of them. This support will be noted in the application file that goes to the Admissions Office.
As has been stated on lots of other threads here recently, there is no such thing as degrees of support, such as full, lukewarm, cold. It is 100% or nothing, and if you have a slot you have 100% support.
This may or may not result in a likely letter. But again, a likely letter is early notification. You are just as admitted to Columbia whether you get a likely letter or not. If you are curious about this, you can ask the coach about the percentage of recruits that have received likely letters in previous years.
Since there is no document such as an NLI to sign, there is really no authentic signing ceremony that can occur. You or your high school could stage one on your own if you wish. The time for this would be after you receive notification of acceptance, either through a likely letter or at the standard notification date for all ED applicants