Regarding the HC versus Asst. Coach question, that is more relevant for the D3's versus D2 and D1. It is definitely a plus to be seen by the D3 HC at one of the events, while the other division HCs tend to lean more on the RCs anyway, so you can gain interest from the HC if the RC likes you. And as others have said, the HF showcase is designed much better for having opportunities to actually talk to the coaches. It is a big plus that HF focuses on getting the coaches and players around each other a lot in the dugouts, on the field. So talking to an Asst Coach is better than not talking to a HC at Showball.
My 2017's experience with Showball is that the coaches tended to hang with each other and not mingle with players nearly as much. And the games were on scattered fields in different locations so it was random luck if a coach/school was at your game. HF had it arranged where coaches rotated fields so there was at least one game where they would be at an interested player's game.
Finally, remember that for the HAs, it is always going to be about the academic scores. If your son has a very high ACT/SAT score or GPA at or over 4.0, put that in the subject line of an e-mail to a coach (W. Jones OF Atlanta 6.8 60YD 33 ACT or something to that effect). My 2017 seemed to get a much higher response rate on his e-mails than other kids he knew by putting a key measurable and test score in the message header. I understand why when coaches say they get hundreds of e-mails each week.
It's going to be an exciting Summer for you and your 2020. Good luck!