Controversial opinion, but I think we've been riding our luck for several games now. Not every game, but I think you can go back to February and say that on the whole, we haven't been convincing. I've mentioned previously that when Johnston came into the team and was scoring bangers from 20 yards, that isn't a sustainable route of success. As it turns out, he's scored fairly consistently, but I still maintain over longer periods it's not sustainable.
People can like or dislike stats in football, especially xG, but it is noticeable that after a period of not giving up very many good chances to teams about 13 games into the season, the quality and quantity of shots against us has slowly risen fairly consistently, to the point where we are conceding an xGA (xG Against) over 1.5 - i.e an average team would score either 1 or 2 goals against us per game. This is in contrast to earlier in the season when we generally kept teams under 1 xGA per game.
At the other end of the pitch, we've also seen our ability to consistently create good opportunities falter. Like I said, Johnston (or anyone else) scoring 20 yard bangers is great but it shouldn't be how we score a majority of our goals.
I'm out the country so can't verify this on mobile, but I would guess our off the ball intensity is suffering as fatigue sets in over a long season.
Personally I think xG is often overexaggerated by fans/media etc. However, I fully agree with your points. Even without stats involved and just looking at it from a fan perspective, we've been pretty poor for a while now. It's pretty obvious and our lack of wins have proven this. I'd also argue that without Johnston in the team we'd probably be outside of the play-offs right now which shows how dependent we are on him. In terms of luck, you could argue that we've been very lucky to draw against Milwall, Watford, and Stoke which is rather telling.
Overall, I really like Corberan and think he's done a great job. However, I also think we're experiencing some dejavu. Last season around now we really struggled albeit with a worse team. My personal theory is that Corberan struggles against certain types of teams who play very defensively and to a degree, cynical football. This is why Eustace always had his number. Earlier on in the season, the clubs near the bottom (and even those looking at play-offs/promotion) play more open football. However, in the final 10 games or so, the strategy becomes more cagey. As a result, we often look worse. Unfortunately, there's a big group of team either going for promotion or survival which means the chances are, we struggle. The benefit of this is that our next two games are against teams with nothing to play for. Combined with being at home, we should hopefully win those two games which should see us into the play-offs. If this is the case then whatever happens from there, it's a case of job done for Carlos this season. I just hope my theory is correct.