I believe Dave and Geep are correct here.
How Engagements are described in the rules:
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Assaults are not necessarily face to face or toe to toe, simply close enough that individual infantry are able to engage in the fight and make use of their small arms, grenades and other short ranged weapons.
The important part of the rules is from 1.12.5:
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If all of the attacking units directly engaged in the assault are killed then the assault has stalled and the defender wins (go straight to 1.12.8 ). "Directly engaged" means being within 15cm of a defending unit after charge and counter-charge moves have been completed. If even one of the original attackers that were within 15cms of the enemy survives, then the attack has not stalled.
In order to qualify as an "
original attacker" (and actually roll dice), the attacker has to have Line of sight:
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but are within 15cm and have a line of fire to the enemy can use their firefight value
Supporting formations also require Line of Sight:
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Calling on support allows units from other formations to attack with their firefight value if they are within 15cm and have a line of fire to an enemy unit directly involved in the assault.
Basically, every part of the engagement rules mentions line of sight/fire at some point. Los is required by any surviving attackers to allow supporting fire. If the attacker was never in position to actually attack the defenders (due to no Line of Sight) then the Engagement stalls before any dice are rolled.
There is no mention of an Engagement representing pressure or proximity. It is simply Line of Sight/Fire and 15cm.
I'll come up with an FAQ to clarify the situation.