Hi, I thought of sharing my findings in case it is helpful to anyone in the future. Thanks to this old post which was golden. Based on my tests:
All you need is starting a melee attack against an enemy to permanently add it to the list of threatened targets, without needing to complete it.
So you can quickly start & stop an attack against each enemy, or just complete attacks vs various enemies over time, and you'll permanently get the -5% for each.
If you have engagement slot(s), an enemy is added when the green arrow moves to it, which sometimes lags behind the attack when switching targets.
Engaging an enemy will automatically add it to the list, without needing to start an attack against it. So more engagement slots help but are not mandatory, not even 1.
If you are always attacking the same enemy, having several engagement slots will automatically give you -5% * N when such slots engage (assuming the enemies are not immune to engagement).
If you are quickly switching targets, the more engagement slots the faster you will get all nearby enemies threatened because upon every target switch the engaged targets will automatically change.
You can memorise which enemies you have already engaged or started an attack against, and spin around yourself switching your starting attack & engagements against whoever is missing.
Being CC'ed (stunned, paralyzed, knocked down), switching weapons or moving will empty the list but using consumables won't. CC'ing a threatened enemy won't remove it but getting too far away will.
If you become invisible, if the enemy doesn't go too far away it will remain threatened.
For a solo run this could be very powerful, as you could end up being surrounded by let's say 8 enemies (-40% recovery) or induce it by using pull effects like Into The Fray, Pull Of Eora, Implosion Charge...
Edit1: if without Mob Stance you have threatened so far N enemies (which you can speed up with Guardian Stance + switching attacks) and you then activate it, the -5% * N recovery is obtained instantly. It makes sense as the threatened list is unchanged.
Edit2: I just realised Entonia Signet Ring's Inviolate (+2 All Def / Engaged) scales with how many enemies engage you, not how many you engage. I find Guardian Stance less mandatory now (except if I need Persistent Distraction), both have their places.