Now first of all, I don't think that Ren was ever meant to be a villain in the same vein as Vader, Darth Maul or Palpatine, because he clearly isn't. From the start it's pretty obvious that he is struggling within himself. He wants to be like Vader but can't and that infuriates him, he is more like a lost child that is being manipulated.
He is not "evil" at least not yet, he is turning evil but is not quite there yet.
if he recognized that one of his troops was on the brink of rebelling, then why didn't he simply have him imprisoned or subjected to interrogation after returning to base?
I don't think he recognized that one of his troops was on the brink of rebelling, he simply noticed irregular behaviour, that's it. When told about a rebelling stormtrooper, he simply put the two together, irregular behaviour before, and now a rebellious troop? Could it be the same guy? Probably.
Though they seemed to have measures in place for this sort of behaviour. That's why the captain told Finn to see her, they simply didn't expect things to escalate so quickly.
Another key illustration of Kylo Ren's incompetence would be noted by his constant temper tantrums/flashy displays of power whenever things don't go his way - contrast this childish behavior with how Vader calmly force chokes individuals who've betrayed him/failed to accomplish their missions. Kylo Ren isn't a teenager - he's actually around 30 years old, but he doesn't really act his age.
Because he is unstable, during the movie there's quite a few times Ren remarks on how he feels the pull to light, while on the other hand Snoke pushes him to the dark side and he idolizes Vader. There's a constant struggle inside him.
You also have to compare their experiences, Ren was basically training with Luke until he got corrupted and turned to the dark side. While Anakin spent years alongside Obi Wan, fought in the clone wars, was slowly seduced by palpatine, then spent a long time serving under him before New Hope even began. There's a huge difference between the unstable and inexperienced Ren, and the completely corrupted battle hardened veteran Anakin. So of course they'll behave differently.
he lost a lightsaber duel to a character with no training and no knowledge of the Force (Rey) despite being instructed by Luke and Snoke (who appears to be an exceptionally powerful dark side practitioner even if he isn't a true Sith).
Well perhaps he wasn't really at his full strength, with a blaster wound in his abdomen, massive shock after killing his own father and the destruction of the big ass weapon. Those things do take a toll on a person, especially on someone as unstable as Ren.
Non-Force Sensitive individuals like Finn and Chewbacca seem to be capable of injuring him, which is pretty pathetic if you think about it.
Chewbacca managed to injure him, after he killed Han. Again, Ren is unstable, and killing his own father is not something that can be done lightly. Even Vader had difficulties finishing off Luke. So it's not a huge surprise that Ren is shocked and not completely himself after killing Han.
As for Finn, again Ren is still shocked after killing Han, now he's also wounded and bleeding out. So no, it's not a huge surprise that Finn managed to hold him back a bit.
his idolization of Vader isn't necessarily logical or sensible (Vader wasn't even the most powerful Sith - his potential with the Force was always limited by the cybernetics sustaining his life, and plus, he famously betrayed Palpatine to save Luke by the very end)
Yeah but you know....Vader was his grandfather along with being extremely powerful. That grandfather bit especially seems to be kind of a big deal to Ren. I'm also willing to bet that Snooke has been manipulating Ren using his relationship to Vader.
Being motivated by rage and hate are good qualities for a dark side user - however, his lack of confidence in his own power and his pull towards the light aren't appropriate. O
Why not? Why isn't his pull towards light acceptable? He's not Vader 2.0, he's a new character with his won traits, personality and flaws. He is not completely evil, that's the whole point. His pull towards the light makes him more interesting as a character, he's not just "evil", he is trying to be evil.
His lack of confidence comes from his lack of experience, again. Anakin was a war veteran, who spent years training with Obi Wan, then decades under the emperor. He knows that he is powerful, but he knows his limits, he knows he isn't all powerful. He has experienced defeat, loss and sorrow.
Ren hasn't. Chances are that he has very little actual battle experience. He has always been extremely strong, Snooke probably making sure he knows that he is powerful. But he has very little experience with defeat. He is confident as long as things go his way but can't deal with someone who might actually pose a threat
Overall, I also feel that the big reveal of Kylo being Han Solo/Leia's child was made too soon in the plot's development - it would have been far more effective if this plot twist (and the unmasking) had been saved for the final confrontation where he kills his father.
I disagree. I think revealing Ren to be Hans son early, worked better overall. Especially when you factor in Rens constant struggle and pull towards the light. At that point the audience still has hope for Ren, much like Han. Most will think that there's at least a chance Ren will go with Han, not many were expecting him to kill Han, because they thought the light had enough effect on him to prevent that.
Now when Han steps on the bridge, we know the stakes, we know the relationships, we can expect something. If he had been nameless and faceless to us prior to this moment, most would easily expect him to kill Han. After revealing his identity, there wouldn't really be enough time to digest that before the killing happens. It would be too much important stuff happening way too quickly.
Looking at your complaints, it seems that you simply wanted a villain who's Vader 2.0. Extremely powerful, confident and all around badass with very few weaknesses. Ren isn't any of that, but that doesn't make him a bad villain, it simply makes him different.
His character is different than Vaders or any other major villain and that is exactly what makes him interesting. his inner struggle, his quest to become evil, all of that makes him an interesting character, and a new character. Something different than what we've previously seen.