I think what we're missing here, is that, generally speaking, the wording on a card will trump a rule in the book.
Assault - As part of a charge, after moving but before making its charge attack, this model can make one ranged attack targeting the model charged unless they were in melee with each other at the start of this modelʼs activation. When resolving an Assault ranged attack, the attacking model does not suffer the target in melee penalty.
Lets run through it.
A model is engaged by a model it doesn't intend to target with an assault action.
Does the attacking model have both its movement and action available to it, and otherwise can it legally declare a charge? Yes.
Declare the charge target.
Is the charge target in LOS? Yes.
Turn to face charge target.
Move towards charge target at up to SPD+3" in a straight line in faced direction, without stopping until the target model is in its melee range, or it reaches its maximum distance of SPD+3".
Did it move 3" or greater? Yes.
(Now we're going to assume it moved 3" or greater, but that it didn't have to move its full distance, because it would otherwise have to leave the melee range of the initial engaging model, and that under some interesting voodoo, the initially engaging model still has it in its melee range, as does the attacking model, so that no free strikes are incurred, and they are still engaging each other at the end of this movement, which is what this is about.)
Charge requirements met.
"after moving, but before making its charge attack" timing check.
"this model can make one ranged attack targeting the model charged unless they were in melee with each other at the start of this model's activation."
Were the assaulter and target engaged at the start of the assaulter's activation? No.
Is the target currently in melee range with assaulting model. (Yes, otherwise it'd have had to leave the initial engaging model's melee arc by continuing up to its full movement until it reached the target, or hit its maximum movement.)
Assault requirements met.
Perform ranged attack, "the attacking model does not suffer the target in melee penalty."
Ranged attack roll.
If hit scored:
Ranged damage roll.
Is target still alive? (Yes/No... now this is where you would otherwise perform your charge attack, and is beyond the scope of this question.)
That seems like it to me.
It seems to squirm around the rule allowing you to make a ranged attack while engaged in melee.
Also, assuming you're still engaging the initial model, it would stand to reason by the charge rules, that if you shoot and kill your charge target with the ranged attack, you can then use a non-charge attack to attack the target you were initially engaging.
This is probably not intended? But from the rules I'm reading about charging and assaulting, seems cheesy, but within the letter of the law.
It seems to me that Assault probably SHOULD be errata'd to include the following. "A model cannot make an assault during an activation it begins engaged in melee with an enemy model." or something similar.
At least it does appear that you cannot use Assault on a model less than three inches from you, which would prevent the 'charge attack', because of the "As part of a charge, after moving but before making its charge attack," part would be violated, given that the model wouldn't be allowed to make the charge attack.
So the cases where a model can move a full three inches through a model's melee arc, to engage another enemy model, without suffering a free strike and dying, and fulfilling all this will be rare at least. I can see it perhaps happening with a non-reach small base model moving through the arc of an engaging model with reach and a 360 melee range, in such a way that the small based model never gets its .5" melee arc in contact with the engaging model. Say... a Rhinodon or Epic Butcher or something. Maybe...