I'm not seeing it. Jaron is not exactly chomping at the bit. He's a non-issue in a year or two after the conspiracy completes. Dandeer is as well. Half the conspiracy involves ensuring that she won't get the Super Saiyan help she'd need to try again, and Jaffur certainly won't be letting his guard down around her. Making him stronger just makes him more capable of causing problems during the operation, and he is under magical compulsions which likely don't care how he feels about Kakara. The invasion isn't going to be a subtle scouting mission conducted by elite infiltrators. Stick him in the hall during the invasion and in a year or two grab him with IT for the plot. Don't risk provoking his mother into changing the scenario. One more mediocre-at-the-very-best and completely uncommanded combatant isn't worth risking a conspiracy that could get everybody involved executed for high treason.
In the order you brought them up:
Jaron is an important part of keeping Jaffur sane and in check after released so gaining influence with him now is worth it for the aftermath. Besides, he is clearly keeping secrets which we should get him to tell us just in case. Jaron is an untrained sorcerer and we have seen already what one of those can do, which means we would be better served if he came willingly, which he would be more inclined to do if he trusts us. Whatever information he has could avoid us last minute complications.
So what if half of the conspiracy is stopping her from getting super saiyan help? How does training Jaron give her more super saiyan help than the one she already has in Berra? Making her act would actually fracture her relationship with him more, which has already cracked after the issue with the nukes.
Making him stronger also makes him trust us more which simplifies the operation.
We don't know if he has magical compulsions. We haven't seen anything pointing to it bar the one about loving his mother, which would be an issue if we were trying to kill her. Which we aren't.
We can't stick him in the hall during the invasión because Dandeer is obsessed with keeping him out of saiyan society.
The conspiracy kind of falls flat if Jaron dies. Jaron, who has already jumped into a fight without any idea what he was doing. Jaron, who just told us his idea of a good person is the one who acts to help if they have the power to do so. Jaron, that does have the power to help and is likely to get stronger by the time the invasión comes. Jaron, who will already know to power up, fly and throw ki blasts by then, enough to fight but not to do so effectively.
As for him being mediocre at best? With the rate he is learning? Throwing ki blasts after just seeing them and learning to fly in a single day, who inherited enough insticts from Jaffur that he knows kung fu and goes for the throat. Not only does that point to him growing insanely fast but, if he fought, there is no freaking way he would do so unguarded as the Scion and his utility would be in terms of morale.
But the point is that, for all that you argue that he wouldn't make a difference in the fight? That is not the point I was making because, as we have seen in the update, he might not fight even if trained for fear of disappointing his mother. It would, indeed, be better if he didn't fight. However, my point is that if he did somehow end in the fight, it would be better if he knew to defend himself until reinforcements came to help him. Because there is a real chance he would get involved, not only because his moral compass and high power level compels him to fight whether we train him or not but because he is also on video beating the crap Tamar's BFF. And while he might be busy hunting Berra and Maya, that doesn't mean he won't reward whoever put us down so it is even likely that Jaron will be actively hunted during the invasión.
