I think I understand your question, and it poses some interesting problems.
To focus on the main point, let's first assume that all LOS is somehow clear and that the AFV is already facing the target(s) - turret spine set NE: pointing along the P10/010 hexside).
You would like to fire Canister ammo at the three hexes joined by vertex T9-T10-U10. None of these hexes contain a KEU, so the resolution against HIPsters and ?s will be made as Area Fire. In addition, C8.41 states that Canister will also attack enemy units in the adjacent hex which lies along the LOF to the target vertex. In the example, this LOF is traced through O10 and not through P10, so the units in O10 will be attacked as Area Fire (in this case quartered since they too are concealed).
Nothing prohibits you from firing Canister at a vertex whose hexes are devoid of KEU (the Area Fire penalty taking the place of Case K suffered by other types of ammo). So go ahead and fire. I do wonder if having no regular target should influence the FP against the adjacent hex (quartered FP for 2xArea Fire before halving again for concealment?), but the rules don't say as much. I don't think it would.
With that, I would say that your basic premise is acceptable. You can fire Canister at no regular target just to hit the adjacent-hex target with Area Fire.
Adding back the obstacles creates a new problem, however. While the rules say that you do not need to have the target vertex in your LOS, you do have to have one of the three target hexes in your LOS - if only because Canister is a Direct Fire attack (LOS must exist for a shot to take place: see Index). Of the three hexes in this example, only T9 could possibly be in the AFV's LOS, but even that looks doubtful. If it was in LOS, then the shot is allowed (LOS to each target is assessed individually as per a Q&A). But I think to qualify as legal, Direct Fire, LOS for Canister ammo must be traceable to one of the three hexes belonging to the target vertex.