None of these apply. Ubel and Land are not connected to powerful people in the Empire, they are their enemies. There was to trial for legal loopholes to be used, no guard was bribed and there weren't any gaps in the security because there was no security at all, therefore they couldn't.
1 ) These are elite forces from a long existing and prestigious organization, not "upright detectives", thugs or whatever.
2 ) This cannot be blamed on a personal mistake. Any half decent organization, be it governmental or private has something called operational procedures exactly because people make mistakes, this is why procedures are created and personnel undergoes training. If you work anywhere with risks, unless you are from a very undeveloped country, you will see it first hand.
Not separating and keeping watch over them is not a mistake, "oh, I forgot about that", is complete retardation. Not taking them to a staffed facility shows that these people have no idea about what they are doing, they are not even thinking. Mages of this level are usually at least a bit smart, this is a very simple mistake.
About prison breaks in our world, it is not completely comparable for three main reasons.
1 ) These are extremely high profile targets who have just been captured by elite police forces. Go sniff on government affairs inside any power nation, be a modern one or even from over 3000 years ago. If you get caught they are not going to leave you like that and talk to a companion as if you were just a thief who stole a apple.
If you are a know terrorist or a spy from an enemy nation, then you attend a place full of elites of your target nation, get caught and escape easily like that, I'll send you a hand written letter deeply apologizing and also publicly apologize. I never heard about someone escaping from SWAT, FBI custody like that. Or any comparable group/agency/force from around the world, pick whatever you like. Note that custody does not mean prison.
2 ) Mages, even more first class mages, are essentially walking weapons. If you caught a terrorist group, would you let them keep all their weapons when you jail them? The mana blocking thing is not enough of an excuse, because even as readers we know that magic makes so many things possible, something that those elite mages should know better then us. By leaving them be, they are undertaking a huge risk since these are not random shit tier mages and you can't be sure that they don't have more allies around.
Even I know how dangerous these people can be, which is why I would do what I suggested on my first post.
3 ) It is not easy to escape from a high security prison, you know, where they send the most dangerous criminals to, like first class mages subordinated to someone you want to kill. All those escapes by relevant criminals took a lot of planning and preparation, it works because the people who administrate the prison do not expect it and usually some guards are also on it, turning a blind eye to obvious signals. You would expect first classe mages to be capable of something, it is completely reasonable.
Seriously, if that is their standard, organisations from our world could likely take out the empire without any magic or super giga warriors even with 1940's technology or less. And I can guarantee you that if they were caught in our world, they would be treated much, MUCH, MUCH worse, even more because they are a prime source of information. I can describe you what would happen, but since I don't know how old you are, I will avoid it.
And I don't watch movies and outside of anime I don't watch TV shows, so my knowledge does not come from fiction.
Like I said, so far the author has been doing a good job setting things up, making these events even more staggering. If it turns out that the people who caught Land and Ubel are actually traitors and they get found out or at the very least investigated for this utterly shameful conduct, it would justify this mess.
