Big questions begs for big answers :)
"If one is skeptical concerning enlightment (because no one besides the buddha and mysteriously ONLY those people in his time got enlightened) does it then even makes sense to practise Buddhism?"
First a small correction: there are many who, through time, had declared to have reached the same enlightenment that the Buddha declared. The Buddhist tradition (literary and cultural) does talk about past and present arahants, those people who brought the buddhist training to it's conclusion during their lifetime. Speculations about who is arahant is a favorite "past times" of buddhists.
As to the question itself, many people see the benefit of a large number of buddhist practices and assimilate them in their lives even if they don't believe they would get enlightened during their lives. So, I suppose it's a personal choice based on other things, not only in the belief of nirvana "here and now".
"The whole point is to escape impermanence and attain the unconditioned, Nirvana. If monks like Bhikkhu Bodhi even claim not to have realized enlightenment how are lay people going to realize this?"
Well, bhikkhu Bodhi is not the only bhikkhu ever lived. He is notorious in the west for his scholar work and social engagement. But being notorious, scholar, or engaged have very little to do with how far one is on the path. Besides being notorious, do you think he is a model of accomplishment?
"[...] but impermance is always used in a bitter way. Yes, we are going to get sick, old and then die at the end. Yes we cant cling to experience, but we can recall positive experiences and they change our brain.. this is why loving kindness works. Impermanence is always exploited to the extreme.."
The bitterness understanding of Buddhism is quite unfortunate and buddhists who take that stance often seem to flirt with depression. But I don't think that's a good understanding of the doctrine. See this thread: Buddhism is kind of depressing.
""well since everything is impermanent I give up on everything and better become a monk." Isnt that true?"
I guess if we stick only to genuine motivations that lead people to ordain, than perhaps, yes...it's a decision to be fully dedicated to the way out of suffering -- and, more often than not, to help people as well to ease their own suffering.
"If you go by that belief it's quite bitte because monks that are long in their business havent already realized nirvana/awakening and the chances are high that they wont. So why then belief in Nirvana and Enlightenment?"
I think it's safe to say most monks are monks in the formal sense: they wear robes, and they are affiliated to a sangha. That says nothing about their day to day. Are they committed to training the doctrine of their own religion, do they even know what these are and how to practice them? Or are they doing something else? I think, more often than not, it is the latter case.
And then, there's another kind of monks, people who are often hard to encounter. And these can be quite special people.
"Couldn't it be the case that Buddhism is just another fairy tale that want's to trick people, to live poorly and to keep quiet in tough times due to getting bad karma otherwise?"
It could. Are you sure, though?
"If enlightenment is just about grasping the 3 characteristics and experiencig them during meditation then even an "evil" person can become enlightenment. Why does it take so long to become enlightenment if it is just about seeing the 3 characteristics?"
Perhaps a more informative description of Nirvana (arahatship) is the destruction of ten fetters. Studying what they are might give a glimpse on how hard it is to accomplish this goal.
Some Mahayana sects might add a few extra requisites to characterize final liberation.
"What if rebirth truly doesnt exist? The whole point in striving for enlightement is then pointless."
Maybe enlightenment would be pointless in this case, but would the entire teachings as well? After all, there's this very life still to take care of.
In one of the discourses, the Kālāmas put forward similar questions to the Buddha. While talking to them, the Buddha found the Kālāmas agreeing that good deeds lead to welfare and that bad deeds lead to harm. In other words, that being a good person has benefits in this lifetime. Then, using a hypothetical person who has strived to become greatly virtuous, he offered the following argument:
“This noble disciple, Kālāmas, whose mind is in this way without enmity, without ill will, undefiled, and pure, has won four assurances in this very life.
“The first assurance he has won is this: ‘If there is another world, and if there is the fruit and result of good and bad deeds, it is possible that with the breakup of the body, after death, I will be reborn in a good destination, in a heavenly world.’
“The second assurance he has won is this: ‘If there is no other world, and there is no fruit and result of good and bad deeds, still right here, in this very life, I maintain myself in happiness, without enmity and ill will, free of trouble.
“The third assurance he has won is this: ‘Suppose evil comes to one who does evil. Then, when I have no evil intentions toward anyone, how can suffering afflict me, since I do no evil deed?’
“The fourth assurance he has won is this: ‘Suppose evil does not come to one who does evil. Then right here I see myself purified in both respects.’
-- AN 3.65
Basically, he is saying that being virtuous has, at least, benefits here in this very life. And if turns out there are intrinsic harmful effects of evil actions, one won't suffer them because one does not do them. Now, if there is another life after death conditioned by the previous one, the effects of bad actions won't drag one to a suffering realm. So, in essence, being virtuous is a win-win situation.
Notice the Buddha is not asking the Kālāmas to believe rebirth at face value. He is just teaching them to be intelligent with their choices in a situation where they don't have the knowledge to be certain about things they are uncertain.
"Most people and monks dont question every doctrine. They use it as Thanissaro Bhikkhu said as a "working hypothesis" and therefore filter out any contrary evidence. What I mean by this is that they are succumbed to an argument of authority."
I think there are people who take what the texts and their teachers say as "working hypothesis" and put them to test. There are also people who take them as authority. And there are people who take them as authority and filter out contrary evidence.
Perhaps the majority is in the blind belief side. Then again, for one example, I read it's pretty much the same scenario in the mathematics circles when Bertrand Russell was studying, when no one questioned the foundations of mathematics or saw it's fragility. It was pure blind belief.
Cracking wrong beliefs is not an easy thing, anywhere.