You probably won't like my response as it is outside the box and non-canon (so far- show) 50-70's sci-fi-ish. This post is buried so I'll drop it:
MAJOR SPOILERS
Young Ned is not totally old Ned - he's been changed, just like Jon Snow has changed -- as a result of death and as a result of entering into minds people change, in an as needed shape-shifting way - by superior beings, Gods of sorts. Everything Ned remembers as experienced is not true, well, those memories are different from how he first experiences events -- until the last time those things happened. In the bigger scheme, events and people are recycled under new revised rules by competing parties, let's just say Greens versus Reds, until it is done right to satisfaction or one of the competing parties realize a countermeasure has been introduced into the loop revising past histories. King's Guard and knightly rules of fighting being one such moral law - and something Howland was not subject to nor did he have knowledge of as he was not a knight. sometime in the past it was an introduced value that knights don't stab people in the back when fighting. I suppose this was introduced after Jamie killed Aerys--very very soon thereafter in fact, as it precluded Ned or his hoonorable knight buddies from fighting Arty in a dishonorable way. This is an infinite loop of sorts, as a plot device-- including the potentional for a butterfly effect- which is why I suspect Lyanna died-- Bran caused Ned to hesitate (and Bloodraven realized it exactly the moment it happened).
The problem Ned has - future Ned specifically, is that his head has been re-looped so many times to change how he responds in the past history he begins losing himself and his younger fun-loving tough arse qualities -- with each iteration of revisement he is less Ned- a REVISED version of himself (rNed 2.0 moving towards Hodor 8.1). If this keeps up Ned will be Hodor-like in the end. --
Within the warg society itself it is known to be an especially heinous act to enter into the minds of humans -- this itself being a *culturally introduced value at some time as somewhere in history a wrinkle was caused with significant outcomes. Maybe that outcome is Hodor? Maybe Hodor was hisself a knight? Maybe he was the son of Lyanna, Ashara, or Elia? He is one of those certainly. In fact, I suspect he is Lyanna and Rhaegars son - and has been purposely warg'd to prevent himself from becoming tptwP -- does that blow your mind? It should. GRRM is a genius. Cat's out of the bag.
I think HBO needs to do a better job at explaining this warg thing - the outcomes of warging humans and what that means -- maybe they will- most likely in the final episode of season 8 there is that reveal. Bran, unknowingly, is diminishing the mind of his own father (well, Ned is his biological father, yes). Bran is a genetic freak of evolution - far out on the bell-curve of probability-- this is why BR has been awaiting him so long. But he has happened- thus 300AC and all that is that. All the right genetic and environmental factors had to come together for Bran to be born -- a thousand-thousand-and-one generations, perhaps (or whatever that number is).
Greensight is similar in ways to the skills Thoros used on Beric and later on Catelyn as just like Beric and Catelyn lost some or a lot of their minds respectively and in -part is related to factors such as duration of death and intensity of the residual deathly emotional state the Gods either do or don't respond with life. The Gods might not even be real - kinda, most likely they are - but those finely tuned mechanisms don't really matter and probably should be left for wonderment. So, Gods selectively responded to the spoken words of usually some kind of Priest/Priestess to bring those character back -- and there might even be a relationship to the House of Black and White in there somewhere (yes yes there is), although in all honestly I think the relationship in Westeros is found in the new chapter release shoreline cave the Children of the Forest have where Arianne is set in the newest chapter of TWOW--(I might be wrong on these facts as I haven't read it yet). I suspect Arianne bcomes a Green Priestess, similarly opposing someone like Melisandre (although I don't think Melisandre is skilled at all-- she's really a tool). This Whitefish Cave is, like nearly everything in the ASOIAF world, a diametrically (sic) opposite to the functional effects of the House of Black and White which has an end product of death the "Whitefish Cave" or whatever you want to call it name, of the CotF is oriented towards life - so at converse to tHoBW a requires death, WfC requires a life.*
If Bran and BR keep it up more, relooping ToJ, future Ned will eventually be Hodor. Maybe he is? (he isn't- Hodor is the original Jon- Lyanna's son -- Jon Snow is a bastard who later becomes a mortal hero-- and not a label such as bastard as he was before). BR and now Bran are essentially Gods (of the Green side) - and BR is trying to teach a few things to Bran in a way so that Bran doesn't disrupt the past as BR has things in time and place where he needs them for Bran to work those issues - in fact, Bran is beyond even BR's skill level -- Bran is the top of the line, formerly human mortal being now God-ish in ability. The Greatest Greesnseer ever. With great power comes great responsibility.
R+L=H