If this were you what do you think you should do in these situations? Even I, the player, got intimidated by these scenes with all those guns around.
You're up against a formidable enemy who holds absolute authority and power, and who has a track record of hostility and wreaking havoc in others' lives. This adversary knows everything about you and your loved one, and to make matters worst, you find yourself in their territory. What options do you have?
on the other hand, you, who may be strongest in video games(?) but in real life you are just a kid who didn't even finish high school yet. You're lacking information about your foe and their capabilities. even if you have some advantage (Calypso and Gems) to use as a bargaining ship. But in the face of overwhelming power, can you do anything besides relying on plot armor?
Even if you can run to Eternum with your own body, what about your loved one?
Even if you want to fight, do you have enough resolve to burn yourself (and your loved one) along with your enemy?
The fact that he was targeting Alex and not you, despite having all the information, is your only saving grace. Given his personality, it's unlikely that he hesitated to resort to underhanded methods. Perhaps he's unable to leverage the threat of Calypso and Gems against you due to certain restrictions? Or maybe he has his own plan that holds greater value than a mere cheap trick?
for me, given William's personality, the only way to catch him off guard and (maybe) calm him down is to tell him that you will become king of Eternum and marry his daughter. If he didn't get even more pissed off than he already is, at least you can still buy some time with this.