Dave , I can relate to how you think your way through a process . If I become annoying with comments just tell me to back off .
I have not had good luck with the nut and washer type traps that you show for the shower . They tend to loosen with vibration . If you want the flexibility of taking apart without cutting a fernco fitting works better . I also don't think it's a good idea to reduce down for a shower . I didn't read any mention of a vent for the grey tank .
With the shower drain and top of the grey tank being so close in elevation I will be interested to hear if there is any back up into the shower driving with a full grey tank . I have a similar situation and have not come to a resolve yet .
