Ok firstly energybending and spirit bending are not that similar at all, one is a form of bending posessed by the Lion Turtles (or whatever their names are) and later the avatar (aang first), spirit bending is most likely invented by Unalaq and later taught to korra.
Also it would make some sense for korra to completely trust Unalaq as she hadnt seen her father for a while and then discovers all his dark secrets, which is definetly a shock for someone like korra. An uncle can also be seen as a fatherly figure (as seen by Iroh), and she was amazed at his power of spiritbending, so with a little bit of hatred (against her father), impatience (against Tenzins slow teaching especially since air was the hardest element for her to master) and her, well sort of, greed for the power but also to protect others from angry spirits so it would seem sort of natural to go with her uncle.
Also the point of past lives being severed, at first i was very annoyed as to how can they just end thousands of years of knowledge when in ATLA we see a lot of past avatars helping Aang and even the important appearance Yangchen in the comics that followed however it does seem necessary at the same time, because the avatars are all very powerful and knowlegable that when one enters the avatar state they gain all that power, however now it just seemed like adding another avatar to the list, which overall as a percentage isnt much considering Roku technically said Aang was the 1001st avatar which means his power wouldnt add much to the avatar state, and the idea of a new beginning would be good since past avatars may have different/outdated ideologies such as when Roku tried (but failed) to convince Aang to kill zuko (in the comics) but that was just wrong for aang to do, similarly it may actually help the future avatars if they use new knowledge that would be relevant to modern technology, because, hey what good will Wan do when someone has to take down a metal army (this is a joke before you critize the example and Wan is personally my favourite avatar) also i feel like at korra's time the avatar was more iconic and there stood far more challenges as more people had the power to 'defeat the avatar' although such cases are seen with both aang, roku and kyoshi just not as much ( i feel as if they were feared/respected more since korra bought more change and thus more hatred)
Also the idea of Koh making another appearance does sound cool but i dont think it will play much of a role in the plot unless it is to end the relation of Korrasami. And about their relationship isnt inappropriate as katara and aang both (annoyingly) called each other 'sweetie' every time they met (in the comics despite being literally 12/13 years old and although it is a male/female relationship like everyone before-korra is technically the avatar of change right? (i know thats a pathetic excuse but its fine with me)) but i think it shouldnt take up too much of the plot and i fear it may do so which shouldnt become the story overall, however i am fine with it as i didnt really like makorra from the start, mainly because i felt like Mako kind of stole Korra from Bolin and then all his numerous amounts of breakups with korra and asami which makes me think that Mako might just be better of on his own unless they come up of a way of introducing him to the Royal Family of the fire nation.
Finally i think that TLoK opens a lot more questions than ATLA did (which were eventually answered in the comics) but theres just too much to answer and so much that TLoK missed out on a lot especially the characters from ATLA (probably due to the budget cuts by nic :P) so i really hope that rather than starting too many new plots in the comics, they go back and perhaps show a brief of the Gaang's young adulthood or include characters they didnt pay much attention to (like all the power Azula had especially since she was freed in the comics and became even more powerful) or include development in the current story as I felt like although the ending wasnt rushed the plot wasnt continued well i.e they could have had more on the air nomads (especially the children - jinora(and Kai!) and rohan as i think rohan was pretty much a waste of time in TLok (no offence)) or the relation of Aang's three children when they grew up but these are just ideas and are not very good for a plot but just things TLoK could have sorted out more. And the tale of Zaheer should be extended, like although it would be quite random, zaheer just had so much potential and i think they could have done so much more with him
Theres a very high chance that a lot of you will disagree on my views, but its all my opinion so please don't get offended/argue on and on...