Also, it sounds from that description like you responded to a successful Go Aggro with the necessity for a Seize By Force roll, which, if true, is deeply wrong. 'Force Your Hand' means they immediately take damage with no chance to hit back, they could then attempt to attack, but only if they survived the flat damage they just received.

Ok, let me describe the scene a bit more because the idea that it would be a battle where only the player does damage and the enemy threat only receives damage (not hitting back) doesn't sit right with me .

So the savvyhead had a vision of ambushers in wait while he was in the augury tank/psychic maelstrom.

The armored truck is approaching the second checkpoint in a dangerous terrain. The terrain had already been clearly identified as a threat. There was an earlier checkpoint that, after a successful "go aggro" via bullhorn, the thugs simply let the party through without harassment ("go aggro" success responding with the threat "caving and doing what the Gunlugger wants" by opening the gate.)

The savvyhead gets a icy chill and recognizes something from his vision. He tells everyone to hold up. They listen to him and approach the second checkpoint cautiously, stopping short of the ambush zone. The gunlugger is on the .50cal and gets on the horn again, shouting "open the gate and move away or eat lead."

This Go Aggro roll is successful (10+), but these dudes were ready to spring the ambush, they aren't going to cave and just let the party go by without a fight. So, despite the party being wise to it, the ambushers collective "hand is forced" into springing the ambush early. I did, at this point, force a "Seize by Force" roll, this was incorrect, I am happy to admit that. However, I do feel that the story demands that this is not a one-way combat. The party was wise to the threat and as soon as they see that the gate isn't being opened they are ready to spring into combat, but the ambushers are likewise going to fight back, their fingers are already on triggers and such.

As I am re-reading the rules, I do understand what you are saying.  The issue is that the "Go Aggro" rule stipulates "Use it whenever the character’s definitely the aggressor: when the target isn’t expecting the attack, isn’t prepared to fight back, doesn’t want to fight back, or can’t fight back effectively." Using the Go Aggro rule as written, the .50 cal would've simply torn apart the guards as soon as they started to aim their guns, game over. (which is essentially what happened even with the seize by force roll since the range was far and the ambushers didn't have far guns)

So if I wanted to do this over again I would either 1. not allow the party to try to "Go Aggro" because the enemy was already prepared for a fight. or 2. allow "Go Aggro" and not allow the ambushers to fight back at all.

Do you have the hardholder roll for his gang/vehicle even if he isn't present?

I had this situation in my first game a couple weeks ago. The hardholder wanted to assist the other players because he was the only one with a vehicle or a gang, but he didn't want to leave his stronghold as he had just been betrayed and wanted to have a solo scene with some of his gang to reinforce the base.

So he sent five gang members and a truck with a 50.cal to take the Gunlugger, savvyhead, and child-thing on a mission. On the way, they enter an ambush situation. The savvyhead had foreseen the ambush using the augury special move in his base earlier, so he had the gang out and walking along with the truck as they slowly approached the ambush spot. The truck stopped just outside of the ambush zone.

The gunlugger was on the .50cal and he made a go aggro move with a bullhorn. He passed and I responded by triggering the ambush (force your hand and suck it up). The gunlugger with the .50cal rolled seize by force. I used the .50cal as a weapon against the fortified checkpoint up ahead, but I used his success to determine not only the success of that combat, but also to determine the success of the small gang of 5 against the flanking ambushers (small gang vs small gang). So one roll, but two different maths.

I am wondering if I should have allowed the hardholder to roll for his gang (even if he wasn't there) or if it was appropriate to bundle it all into a single seize by force roll. I also realize that I should not have had the Gunlugger seize by force, that my selection of "suck it up" would have introduced combat and that it was redundant to also roll seize by force.

I know this post is super-dead, but my world is most inspired by Samuel Delaney's opus Dhalgren. This book, to me, most accurately depicts the (my) Psychic Maelstrom. Also, this book has the same 'mystery' around what ended the world/normalcy that I bring to my game. Also lots of good anti-normative performance with race, gender and sexuality. Read it!

https://www.amazon.com/Dhalgren-Samuel-R-Delany/dp/0375706682