Does the helper always need the backpack?
No. Either participant can carry it.
Patch 7.0.0 changed assisted reloads: the correct backpack can now be carried by either the weapon user or the assisting teammate.
After Devoid of Liberty 7.0.0, an assisted reload can work when either the weapon wielder or the assisting reloader has the correct backpack. The helper no longer has to be the only person carrying it.
One of the two participating Helldivers—the weapon user or the helper—must have the correct backpack.
Not every support weapon supports assisted reload. The weapon still needs the appropriate interaction and correct backpack.
Editorial recommendation: let the weapon user keep the backpack by default, then allow a nearby teammate to assist when a safe damage window appears. This reduces the cost of assigning one permanent reloader.
Patch 7.0.0 added backpack icons to weapons that require one and a separate icon for weapons that cannot be reloaded. Use these as a pre-drop planning aid.
No. Either participant can carry it.
No.
The rule is official; squad-role advice is editorial.