If the Human Noble is thirty then their origin portrays them as basically feckless layabouts too busy chasing tail and playing at being a warrior in tournaments to take on any responsibility prior to the destruction of their lives. Never being unmarried, and never having taken on responsibility before (and still being referred to by their father as "pup" ), means there's no way Loghain would take them seriously as a threat. A barely of age noble is more unpredictable, but still wouldn't reasonably be able to muster much support among the nobility for a bid for the throne. The massacre is by far the riskier move. Every massacre has surviving witnesses if only among the attacking force. Word would spread of what Howe did. Howe of course was perfectly content to rule by terror. He preferred it that way, in fact. But Loghain actually wanted to unite the nation in support of his regency.
I kind of got that impression from the human noble origin actually, unless you have high cunning or something like that. Aldous treats you like a dunce, nan sort of babies you, Mallol clearly thinks herself wiser than you or as if she can be a guide for you. But on the flip side there are others who treat you as an authority. The guards, obviously. Ser Gilmore is the better example, because he displays some degree of familiarity with you, like you've been friends for a while, yet he still shows respect. The guards playing cards can be handled in a number of ways and they respect your authority. And even the characters who baby you will remark how mature you are if you select the proper dialogue choices. Eleanor will say something like how you grew into a find man when she wasn't looking, or something like that. Nan says something similar if I recall. Rendon is also a good source of your own background. He says your parents are content to let you find your own way. So it isn't like they treat you like a child, even if they do cherish you like one.
Bryce calling him (or her) "pup" is obviously there for gameplay reasons (because the game can't recognize a custom name). But even in a storyline context, "pup" could just be a nickname that stuck. My grandfather's nickname was "Babe" because he was the youngest of his siblings, and yet his own wife called him that, as did her sister, even well into his 80s.
But yeah, chasing tail. Competing in tournaments. That's about right. I mean, mine didn't chase tail. He was a gentleman. But depending on your dialogue, it is possible. Same with the tourney. Dairren confirms that. Rendon also claims his daughter raves about your prowess as a warrior. He was blowing smoke, sure, but there had to be at least some degree of truth to it. He later calls you a fool husk of son still trying to fit into daddy's armor, but that could have just been a taunt.
In all seriousness, it is possible that the human noble was as old as 34. I assume Fergus is 35, and of course you can't be within 9 months of him (it's never implied that you are twins). And it's also possible that the 30 year old human noble was doing nothing but getting educated, getting trained in the arts of war and leadership, and making political appearances such as he did at Lady Landra's party. The one where she got drunk and flirted with him. Was Bryce really much different? He made a visit to Orlais a month before that. I mean, that's just what nobility does. I don't expect him to have a job working in a bakery or on a farm. And as far as marriage goes, with nobility it's WAY different than with commoners. A commoner will often marry for love or convenience. Nobility usually marries for title, power, to join strong noble families, etc. A 30 year old human noble might have been holding out for a good wife, or a noble wife.
And in all fairness, Anora turns 30 in 9:30 according to Eamon's letter, and she was married off to Cailan when she was 25. So it isn't unheard of for nobles to wait longer until they marry. Even Fergus, if he is 35 (he looks 35), minus Oren's age, which is maybe 8. That's 27 when he and Orianna conceived Oren.
And look at how old Bryce and Eleanor are. In my mind it made more sense for the human noble to be around 30 based on how old Bryce and Eleanor looked (though in my case it just so happened to work out this way, I didn't meta game). Even Landra and her son Dairren. She looks old and he looks like he's in his late 20s. Eamon and Isolde, for crying out loud. And what of Bann Teagan? He's old. He's single.
And this actually makes some sense if you look at the timeline. The Orlesian occupation and the war would have an effect on when the Ferelden nobles (then rebels) would have chosen to conceive. And this is also where World of Thedas drops the ball because it doesn't take this into consideration.
For Loghain to dismiss him, he would have to know his personality first. I'm saying that the human noble was an unknown quantity. So it would be better for Loghain to get rid of that Cousland rather than take a chance that he or she will be a loose end.
Well if unity was Loghain's plan then why didn't he rally the banns to punish Rendon first? Loghain could have just called for Rendon's execution and then taken his supporters and wealth and used it to fight the blight. And without the massacre Eleanor and the human noble live on. There is no guarantee that Eleanor would support him.
Plus, Loghain would still have to figure out a way to kill Bryce without raising suspicion.