I've found the rules for the extra crew in a support weapon unit to be rather wonky in game when choosing to represent the model with a crew on foot. The model can be built with either both crew on the weapon platform or with one crew on a separate base.
"This unit contains 1 Support Weapon and 2 crew, one manning the weapon and one standing alongside it."
"Guardian Crew: Each Support Weapon and its crew are treated as a single model for all rules purposes. The crew must remain within 1" of their Support Weapon and cannot be targeted or attacked separately. The range and visibility of all attacks made by a Support Weapon and its crew are measured from the Support Weapon, not the crew."
So what I gather from that is the two models only count as one in the game (one model claiming an objective, one model destroyed, etc.), but the crew model does exist as a footprint, as it must remain within 1" of the weapon. What kind of interactions can this crew model make? It cannot be targeted or attacked, and it cannot be used to draw line of sight. Otherwise, does it exist for other things?
1) Can an enemy model move within 1" of the crew while remaining out of engagement range of the weapon?
2) If the crew is within 3" of an objective, but the weapon is not, does the unit count as claiming the objective?
3) The support weapon unit has the VEHICLE keyword. If a character is within 3" of the crew, but not the weapon, can it benefit from Look Out, Sir?