regulus lacks defensive moves
No he doesn't. Shrapnel mines, mark of the betrayer, and maim are all defensive because they snare. Not to mention that he is a huge pain in the ass to kill after a few points in Sniper Scope he can attack from so far away.
he has below average health and armor gains per level
Doesn't matter since he can avoid damage so easily, and any demigod can overcome any weakness with the use of items. Hell, go Rook, whose biggest weakness is his slow movement speed, and get a swift anklet, buy boots of speed, wand of speed, journeyman treads and an elven cloak, and get a fire TB as your ally and you'll be flying across the map.
Any decent Regulus will almost always have the most kills, and they're the worst gold hogs by far, even worse then Beasts, so it's not like they can't afford things.
his unmodified auto-attacks deal significantly less damage than the other Assassins
I doubt that. Doesn't really matter either since he can maintain Angelic Fury easily with +mana items and of course the uber-OP heart of life.
he has no interrupts
lol the only thing on your list that actually has any merit, he can overcome his lack of interrupts easily enough.
his abilities have high mana costs
wtf are you talking about, he has only 1 ability which costs more than 800 mana, and that's level 3 mark of the betrayer which a lot of people don't even get. Anything else is easily spammed, angelic fury doesn't go up in mana upkeep for some reason so it's easily maintained.
he has the least amount of group support abilities in the game (in terms of buffs and debuffs).
Um none of the assassins except for Torch Bearer have any group support abilities. If you're counting Rook's towers as support then Regulus's Mines count as support also. I would think that his snares support his team too.
In conclusion your list fails, and how about the fact that every single game has at least 1 Regulus, usually 2, undeniable the most overplayed Demigod in the game.
You just don't want to see easy-mode outrange-towers archer boy get his deserved demise.