I'll make a video of the proper way to avoid the HO, or if necessary go nose on to initiate the HO truce. I have mostly mastered this art. The rule of thumb when HOing is not allowed is, if both players dont initiate a HO truce by both getting around and pointing nose on each other relatively at the same time, than its usually not a HO because the others excuse is they are trying to create an early angle that could benefit them in the fight. Therefore you should take the shot to avoid their angle set up. If you both secure the truce knowing youll go head on, then the angle will be adjusted during/after the pass leaving it up to the next maneuver to determine the positional advantage of the fight.
I know you're clued into DA and 1v1's so let me expand your MNM awareness so you can put it into perspective. Also not all MNM attendees are clued in about the 1v1 dueling/acm culture.
In DA or MA 1v1's there are two basic styles: kill or have fun.
Kill takes any and all shots (HOs in MA), often front quarter crossing shots (DA). It's about the kills.
Have fun holds those front quarter snapshots and generally works to get the kill from behind the 2/10 or 9/3 lines. It's about the flying - that "HO truce" you mentioned.
Now MNM is unique in that it's not 1v1 in any sense. It is a furball pick-fest. What complicates this is the rule of "no hoing!" with no clear definition of what a HO is - so some have tried to make it 2/10 or 3/9 shots as being "approved." I would say there are more MA types than DA types in MNM, so that lack of clarity or experience about HO vs front quarter has an effect.
Of course, because it's a furball pick-fest with reduced vis and icon ranges, oopsies do happen. Some are more prone to oopsies because of connection differences or perspective perception of the enemy plane. Those that skim off the 1/11 to 2/10 lines are what I label "sketchy" front quarters (and I generally avoid taking them.) I also call them out as a reminder to check firing perspectives.
It has taken a while to shift from my "have fun" approach to "kill," to "not too sketchy kill" because it's a furball pick-fest, not a nice 1v1.
Simply put, as some have said, it's a true HO if both can reasonably get guns on. But, again, that 2/10 or 3/9 "suggestion" from the rule-maker is what causes it to get blurred as it creeps into the sketchy front quarter 1/11 sector. And me calling these out on text 223 seems to upset certain egos, while calling them out on vox 223 gets a pass.
Also, most are tuned vox 223, even tho' not all players are tuned to it. Some of us are also tuned to ch223 text - almost non-existent ch2 or ch200 text. People are too busy flying (air spawns rule!) to type much, I cannot speak to how much vox 223 (or range vox) is used, but I'm guessing quite a bit.
So, yeah, let's see how you might define things.