TacticalBarbell.com

Priority is currently in order
Strength and adding a touch of weight back on after BB (lost about 1.5kgs)
Boxing Club***
Keep aerobic base honest as I have a trail race coming in 16 weeks and will transition into Green assume black is the right option

Question 1: if my boxing class or class are 60 mins of warm up, circuit drills such as heavy bag, core work, burpees, pad work, Shadowboxing and a few more drills is this covering Endurance, but surely not to the extent of a LSS 45-60 min run, is this class able to hold aerobic levels honest until I chose to make them a priority again in 6-8 weeks?

Question 2: would a hybrid system of such work better where I did say,
Week 1: 2Es (including a longer run) 1HIC
Week 2: 1E being the boxing club, 2 HICs
Week 3: 2 very short HICS no classes

I have read that KB states certain HICS, such as fast 5, boo etc are better for aerobic then say HB resets or 600 reps

Little lost as to how to plan it all out

Boxing generally would fall into HIC work if you want to include it. Repeated rounds of varying intensities will put you outside the aerobic zone.

In the end do what works for you though. I count my sport work (BJJ and Muay Thai) completely seperate to my Tactical Barbell (TB) training.
I find that works better for me and I just drop a session here and there when I need to recover.

The main issue is your goals are a bit all over the place. You have an event coming up in 16 weeks (trail race) but your goals mention boxing and gaining weight. I personally would be training for the event with some supplemental boxing on the side (for fun) and some easy strength work whilst you focus on that race event.