The fact it took him 30 years to twig to the fact his son wasn't right in the head tells you a lot about Harrison.
Oh, he'd known about Caleb's mental instability for sixteen years by
Sword of Sedition (Caleb was diagnosed c. 3118, when he went into AFFS training, per
Fortress Republic - Julian remembers meeting Caleb for the first time when he was eleven, so c. 3118, and "shortly before" Caleb went for training, in
Sword of Sedition), and that it's not a condition that's going to get better. But, as seems to be typical with Harrison, rather than make a choice between actually getting Caleb help and trying to teach him art of rule, or formally disinheriting him and declaring Julian his heir, he buried uncomfortable facts, waffled, vacillated, and in the end got the worst of both worlds.
Harrison was using the visit to Terra to introduce Julian as at the very least his right hand man and possible replacement. Of yourse not including others into his plans was a major misstep. As you said Aaron who is very capable in seeing the bigger picture had an inkling that Julian was more then just the Prince's champion but other then that no one seemed to see it (well except Gavin I suppose)
I don't disagree that was his intent, but it was very much too little, too late, because Harrison didn't do anything
else to make sure Julian was set up to take over if anything happened to him. And if he's starting the process of doing so
that late - well, I agree it's in character for Harrison, given his apparent refusal to make a decision unless cornered like a rat in a trap, but it's also a profound abrogation of his responsibilities as a ruler.
What we do know is that the Fed suns set aside some military hardware (in SoS Julian mentions how basically every house sret aside perhaps 10% of hardware to the side just to feel safe) but in terms of actual training they fell deep into feudalistic traits while also seeing a rise of social generals something usually only associated with the Commonwealth.
Not just that, Harrison and Yvonne very deliberately crippled Mechwarrior and aerospace pilot training for the AFFS, per FM 3145;
Raising trained MechWarriors have become a critical issue for the Federated Suns in the last two years. The post-Jihad disarmament saw the Suns slide back towards its pseudo-feudal roots, with the few open ’Mech slots in the AFFS filled mostly by scions of the established MechWarrior families. If your last name was Zibler, Dixon, Watters, or any variation of the March leadership you could be assured training as a MechWarrior, no matter how poor your skill might be. And if neither of your parents was a distinguished warrior, then the chances of receiving a coveted BattleMech assignment were vanishingly low. By the time of the Blackout, the pipeline for new recruits had slowed to a trickle. After Grey Monday, First Prince Harrison’s attempts to keep the peace kept MechWarrior training still constrained. It was only with Caleb Davion’s rise to the throne that academies were encouraged to increase their class sizes to help fill the rapidly expanding AFFS. It was also during Caleb’s reign that the Training Battalions were reestablished, after more than fifty years’ absence.
[...]
The Suns’ aerospace service faces equally grim prospects. Never fully recovered from the Jihad, aerospace training in the AFFS has barely kept pace with the demand
for quality pilots.
As well, we have Harrison - in one of those fits of whimsy, like cozying up to the Ravens, that proved disastrous for the Suns - decommissioning "most" of the federal guard 'Mech forces, which could - if he'd actually given a damn about doing his job properly - have been a good way to maintain reserves of trained mechwarriors and maintain the infrastructure necessary to expand the AFFS at need.
(honestly, I find myself increasingly coming to the view that, while personally reprehensible in many ways and not a
good First Prince, Caleb wasn't a particularly
bad one either - something that ER 3145 supports, given its description of Caleb's record in command as "competent, if not spectacular" - and mostly just had the bad luck to be in charge when the bill came due for Yvonne and Harrison's incompetence)
