And because his powers got countered by just one of Brandish' subordinates is exactly the reason as why there are most likely other people who could get out of it too. Dimension-based magic is in fact not that rare. Erza for example uses it too. She stores all her armors and weapons in an alternate dimension which she then summons and puts back. Erza also once blocked a strike that cut through dimensions. And who was the one who struck? Ikaruga. And as you said then you also have Minerva. Stellar Spirits then continuously hop dimensions, so there are quite a lot of people I imagine that could potentially escape. Don't confuse Naruto's space/time ninjutsu with FT space/time magic as they approach those concepts completely differently. So no it's not that hax, the main problem was to get all of the people that Jacob zapped back safely.
And it's a simple equation and I don't even get why you need to go into the that person > this person road and start making baseless assumptions that now Gray can become the ruler of Tartaros. Gray already met his father, he fought him, he beat him, they came to understand one another and Gray made peace with it all. This was a year ago, Gray has become much stronger since then. So Neinhart summoning Silver would be stupid as Gray already went through all of that. He thought his father was dead and then he suddenly had to fight him. So an obvious fake Silver popping up is really not going to shake Gray a 2nd time. Lyon then has no connection whatsoever to Silver, so he will have no issues of having to fight him. Silver's psychological impact would be 0%. Ultear on the other hand, well in the least Lyon still had some strong unresolved issues about her and both Gray and Lyon knew Ultear. So her psychological impact would potentially be way higher. And because you want to pit characters against one another, the correct question would be whether Ultear is stronger or not than Silver. Now I'm not even going to reply to that, however Ultear was said to have been powerful enough to be a wizard saint, so she is very strong. So if you count all that up Ultear would by far be much more ideal for the situation than Silver.
And it's you that isn't getting what you are exactly saying yourself. This is a fictional story, everything is orchestrated, everything. The people, how they look, how they talk, how they act, how they live, what they do, what is happening. That is what makes this a story, there is nothing that has ever happened that was not intended to happen. That is the plot. Now people who talk about "plot armor" don't realize that they are implying there is something outside the plot. It's non-sensical. Of course characters get saved by the plot, because it's the plot, just like how people that die also got killed by the plot. Why? It's the plot. The better a writer is, the more he can keep up the illusion that his story isn't fabricated and that it exists on itself and that everything is the result of decisions, chance, luck etc. of the characters themselves. It's when that illusion gets broken too much people start talking about "plot armor" because they see things that are too lucky or too unrealistic for it to have happened in an objective, independent world. However finding that the illusion gets broken too much is something personal and extremely subjective. Why? Because it's still very well the same plot regardless of who unrealistic something might be.
Hence "plot armor" is the biggest BS concept in the history of storytelling and seems to be largely used by people who just want to find an excuse to complain about something, but they can't exactly support their complaint, so they pull the "plot armor" thing. Do you know what you then actually say? This fictional story is...a fictional story, and I'm going to complain about it. Read any fictional series and you could find hundreds of examples of things and events that could have been handled way differently, but that would make for a boring story. No there is no such thing as plot armor, only a plot.
I prefer mine U_U
And I'm not surprised. From the perspective of a manga colorer, FT panels like these are near perfect.
Now you're just thinking too much. He wasn't drowned yet in the first panel, he was still falling. What you see below him is the surface of the water and in the next panel he crashed into it, hence all the water is splashing upwards. It's a common way to portray in short intervals different events and people that happen at the same time: people are cheering; Neinhart is falling; other people are cheering; Neinhart is getting closer to surface; other people are cheering; Neinhart hits the water.