Fun idea! I like that it makes both losing models and keeping them both desirable results. It does seem like it could stand to be simplified and polished though. At a glance...
If a unit embarks on the Mega-Shokk attack wagon in the preceding movement phase, this weapon must be fired in the shooting phase.
What happens if you embark but don't have any targets in line of sight? What if you start the game with units embarked but don't embark any on turn 1? I'd probably just reword this to say, "This weapon may only fire while at least one unit is embarked."
Once this weapon has fired, if the target unit was not destroyed, immediately place any surviving embarked units such that at least one of their models is in base contact with the target unit. If the target unit was destroyed, place any surviving embarked units within 6" of where the target unit used to be. These units count as having declared a charge in the Fight phase, and may not shoot in the shooting phase.
There's a lot of complication going on there. For starters, I'm not sure delivering a unit all the way into combat with this rule is a great idea. Automatically getting into combat right away is kind of taboo at the moment. Units have been nerfed severely for doing so. Some armies pay a hefty 3CP just to deepstrike a few inches closer to the enemy, and then they still have a significant chance of failing a charge roll. People spend points on units and gimmicks that make it more difficult to get into combat after deepstriking or zooming across the table. You could tear apart entire castle lists with a single squad of boyz and this model. Creating a unit that can automatically deliver units into melee from across the table might be impossible to put a solid points cost on. You potentially autowin the game against any list that can be shut down by having one of its key shooting units tied up in melee each turn. So you either make this thing so expensive that the ork player autoloses by taking it, or you make it cheap enough that the ork player can potentially autowin by fielding a couple decent units alongside it.
Setting balance aside, this ability could be hard to resolve. You can potentially fit enough models into this thing to make it rather difficult to place them all within a 6" area of the enemy's last location. Sure, 12" diameter is a decent amount of space, but what if you shot my rhino with a
tac marine squad standing on either side of it? It would be very easy for there to simply not be enough space for the surviving embarked units to be placed.
If the target survives, I don't have to be wholly within a certain area, and you don't specify that I have to stay away from enemy units. So as you've written it, I could shoot this thing at, for instance, a Leman Russ, fail to kill it, and then daisy chain a bunch of models such that I tie up every tank in the guard player's army.
My suggestion is to change it so that you resolve any deepstriking first, handle it as normal 9" deepstriking, and then have the gun's shots resolve against the unit closest to the deepstrikers and within
LoS of the wagon. You might consider changing your rules such that only a single unit can deepstrike in this fashion each turn.
Basically, it's an extra shokk attack gun that doubles as an extra Da Jump that can be performed on the same turn as Da Jump. If this thing is cheap enough, people will take 3 of them and use them to deepstrike their entire offense on turn 1. The vehicle itself is tougher than a battle wagon but nowhere near as durable as a knight. People can plan around maxing out the number of models embarked on a given turn meaning it's going to average 10 shots each time it fires. Shokk Attack Guns are what? Heavy
d6 for an average of 3.5 shots?
As a starting point for this thing's cost, I'd take the cost of a battle wagon or gorkanaught (whichever is closer to this thing in terms of wounds). I'd add the cost of 3 shokk attack guns to it (because 3.5 X 3 is close to 10). I'd toss the cost of a wyrd boy on there as a loosey goosey approximation of the cost of Da Jump each turn. Then I'd take that total cost and add 50% to it because of the extra benefits it has (extra wounds compared to the base vehicle, much smaller chance of failing to fire than of a wyrd boy failing Da Jump, etc.). You'd have to test it out a lot from there. You'll really need to adjust the "launched into melee" thing though. Automatically delivering even the humble gretchin into melee can be an autowin against certain matchups, let alone auto-deliverin gmeganobz or tank bustas.