Your assuming that each avatar has to use their immediate predecessor as their primary source. Yes, Aang primarily talked to Roku, and Korra has primarly talks to Aang (when she talks to anyone). But that's just two out of how many hundreds. The avatar appears to be able to contact ANY of his/her predecessors. If the next avatar doesn't like what Korra has to say, he* can simply listen to Aang, or Roku or even Kyoshi. I don't think there is a hard and fast rule.
Speaking of previous Avatars, anyone else think Korra will eventually have a session with what's his name, the last Water avatar. Beyond the fact that he presumably can understand whatever unique perspective water avatars have (in much the same way that Aang consulted the last airbender when he was having trouble with the idea of killing Ozai (he didn't take her advice, but at least he thought her advice might be worth something). He might have some info that was of manifest use to Korra RIGHT NOW. Since, if I recall he was a Northern Water Tribe Member, he may have some inside info Korra can use (it's probably out of date but still) And if Uniloq is genuinely Northern Tribe nationalist (as opposed to simply trying to aggrandize himself alone i.e. he does not care about his northern subjects any more than the southerners he is trying to conquer) That guy might be the only person Uniloq might actually obey (assuming that while in avatar state it is possible for an Avatar to channel his or her precarnations voices so you KNOW it's one of the others talking.)
*The avatars seem to alternate genders with respect of same tribe members. The last Airbender avatar before Aang was female, the last Water before Korra, male. So I tend to think the next Avatar after Korra will be a male Earthbender, followed by a female Firebender.
"For I have heard the Song of War, and to me, it sounds just like "Please, Mister Custer.""
Cogito Ergot Sum, "I think there is something wrong with this rye bread." (Aplogies to Seafrol.)