For Team Rocket in BW, Meowth traveled with the group as a way to gain their trust and carry out their plan, so I wouldn’t exactly call that being inconsistent, as he knew what he was doing the entire time. I can agree with your other points on TR though, as I’ll admit that I haven’t watched much of BW due to the quality drop.My guess is that they didn't want performance battles as a part of the Pokemon Showcase because that would make the competition look even more like the poor man's Contest. Showcases were already watered down Contest appeals, so throwing in a battle portion would have made people wonder why they didn't just bring back Contests even more. A battle portion could have made Showcases more interesting, but at the same time, they would have been better off bringing back Contests instead of relying on Showcases.
I wouldn't say that their characters were consistent during BW. There were times where they would go back and fourth between their comedy relief personalities to their stoic Team Rocket grunt personalities. A few examples were James capturing Yamask, the Litwick episode and the whole Meowth traveling with Ash's group mini arc. Not to mention they basically reverted back to their comedy relief role/personalities during the last arc of the series. I don't know if I'd say that Team Rocket were at their lowest during XY, but I definitely wouldn't say that they were consistent in BW.
I'm not sure if Goodra had enough character buildup for its evolutions. They rushed through Goodra's development too. There was slightly more of a point for Ash to get Goodra that fast for his Gym battle with Clemont and to save its home, but I'd say it was still undeserved like with Noivern. People often complain about Greninja taking up the spotlight and while I can understand those complaints, both Goodra and Noivern are by far the biggest problems I have with Ash's Kalos team. That and the fact that he had an empty slot for most of the series for no good reason. I wouldn't say that its one of his most forgettable Pokemon. I kept having to remind myself that some of Ash's reserve Pokemon exist during BW, but I never got to that point with Noivern. It was handled completely poorly though.
I'm not sure if Trevor could be considered one of Ash's rivals. They only battled once and that was to calm down Trevor's Charmeleon. They seemed to be going for more of a connection between Clemont and Trevor at least during the Summer Camp arc, but that didn't really go anywhere. He appeared in the Kalos League, but there was no indication he had badges before and it was only so Mega Charizard X could crush Mega Charizard Y. He was just bland and forgettable. At least Tierno always came off as energetic and his dancing gimmick made him memorable. Plus, I give the anime credit for giving him a team that actually made sense with his goal, which is more than I can say for the X/Y games.
As for Alain, I liked him at first, but he just became so overpowered by the time he appeared in XY proper. Going head to head with Legendary Pokemon and defeating ten Mega Pokemon in a row was so overkill. They had to make him a challenge for Ash, but they just went too far. He's basically as overpowered as Tobias was, but without the in-universe justification to make it understandable at least for me. I can buy that Legendary Pokemon are all extremely powerful in the anime, but a Mega Charizard X doesn't have that same kind of explanation. I don't know if I hate him that much. I like his friendship with Ash and how much cares about Marin, but he is definitely one of my least favorite rivals at least.
Honestly, I never quite understood the complaints people had about Serena's battling skills. Prior to SM, she showed the least interest in battling compared to all of the other female traveling companions, but I didn't think that in itself was a problem. Battling is a big part of the franchise, but I don't think that having a main character who isn't interested in battling is necessarily a bad thing. Granted, there are still some issues that can be taken from this, like Serena going into Contests as a way to practice for Showcases despite how the two goals require drastically different skills, including actually being good in battle, and not having any female character actively into battling can be problematic, especially when the only competition the main female lead took part in was basically a glorified popularity contest. Still, Serena didn't really want to battle, so her lack of skills doesn't seem like that big of a problem.
I'm not sure if I'm thinking of a different battle than the one you're referring to, but the first instance of Ash spamming Flame Charge like that that I can think of is his Gym battle against Valerie. He kept using Flame Charge so that once the effects of Trick Room wore off, Fletchinder could be even faster than before and win the battle. I could be forgetting another instance where he just spammed Flame Charge like that though.
With performance battles, they could’ve done something like having them only occur during the Master Class or specific showcases or something like that, just to spice things up.
In regards to Serena’s battling skills, I wouldn’t have minded except for the fact that it seems like she can’t defend herself fully when alone, as shown in episode 56 where she almost lost to TR. The fact that she had to rely on Bonnie, someone who isn’t even a trainer, in order to beat them showed that she couldn’t perform basic battling, something every trainer should be able to do regardless of their career path.
For Fletchling, I’m talking about episode 35, Battles in The Sky, where Fletchling evolved into Fletchinder.