Why must someone strive for something which is everlasting, e.g. Nirvana?
Very good observation and good question. The answer is: you don't have to! There is no "must".
But some people want to! Some people want to find everlasting ease and peace. They are tired of troubles and don't want to hear about any solution that is only temporary and after some time the troubles will re-appear. They want to stop all future troubles once and forever. They keep suffering that life never gives them a break - and they want to find a permanent solution.
Other people suffer for a different reason. They don't worry that things are impermanent, but it really bothers them that everything is wrong and fake. They very much want to find a way of existence that is perfect and real.
Yet other people suffer because they feel they are not free. They feel that their life is constricted by all these externally imposed limits, and that they don't really live, they just tolerate and wait - all their lives. So they want to find freedom.
These things really bother some people. If you don't have a problem with freedom, you can ask someone who has it: "but aren't you already free? aren't you already doing what you want?" - and he will look at you like you're crazy. Same with perfection, some people don't think this world is wrong. Some people actually think this world is totally alright the way it is, with all its trouble and absurdity - they find this imperfection gives them many possibilities to use their skills and take advantage of situations. Finally, there are people who are Okay with impermanence. They just don't expect permanence. In fact, I knew people who were glad to finally die, because by their age they were actually getting tired of living.
And yet there are many people who are really bothered by these kinds of things. In fact, Buddha was one of them before he "awakened" and "saw" how things work.
What Buddha found is that it's all in our mind, it's all about what we're paying attention to. If we have an idea stuck in our head that "things should be this way", and we keep obsessing over the thought that "things are not this way" - we can drive ourselves mad and suffer endlessly.
But it's not enough to just close one's eyes and pretend that you don't see all the issues. That's not the real solution. The real solution, as Buddha realized, was to look inside the issues, and realize that the issues themselves are mind-made illusions. In order to see that, one must understand how conceptual mind works, how it creates these illusions. (This is called "wisdom".) Once you understand how mind works, you no longer attach to any of mind's creations, you don't take any of them too seriously. (This is called "dispassion".) Since you no longer attach to mind's creations, concepts, positions, you are never in conflict with how things are - so you never suffer. (This is called "cessation".) This state is unconditional and "everlasting" in the sense that no matter how things change, no matter what happens, nothing is ever "an issue". You don't need anything to be a certain way anymore, everything is always perfect just as it is. (This is called "suchness".) And because you're now free from attachments, positions, desires, and fears; free from all boundaries I say - this is called "liberation".
Now, I think that it's not good to base one's goal PRIMARILY to self oriented ones, i.e. only achieving enlightenment. I know that this is not a good motivation and that one rather focus on others or on much more measurable goals like.. meditating twice per day or showing forgiveness or to apologize for one's wrongdoings.. Agreed on that? I think you all do but still the main goal in Buddhism is to escape Samsara and the cycle(s) of rebirth.
This idea of "escape" is based on aversion to this world, and on a rather painful attachment to a vague idea of some imaginary state when everything is perfect, forever. That's not what Buddhism is actually about, however. In fact, that idea of escape is exactly the samsaric thinking. Instead, the Buddhist way to escape Samsara is to transform one's mind and see everything as Nirvana, see everything as Great Perfection.
Which leads to my second question:
Does one practice metta just for the sake to cleanse one's karma, for the wish that the other escapes samsara (which is ok if one literally - and not metaphorically - believes in it) and to not be reborn in this - I'm sorry - "mess" yet again?
Like everything in Buddhism, metta is an interesting trick. The kind of people who seriously seek Enlightenment or Nirvana, are the people who usually have strong aversion to this world. This is true by definition, because aversion to this world is what made them seek Nirvana in the first place. So in one sense this aversion is good, because it kept them away from cycling in samsara while playing games and shedding tears, blood, and breast milk. But because they have such a strong aversion to this world, they can't achieve peace of suchness so now it becomes obstacle! To solve this problem Buddha came up with the Four Brahma Abodes, which are the four attitudes that help one get closer to suchness of Nirvana. But because regular world of normal people is based on the same underlying principles as Dharma (conflict is bad, peace is good, in both World and Dharma - because such is the nature of mind!), this attitude of metta also has a nice effect in the world, by reducing conflict and making people happier with each other.
I think, it is better to stick to an agnostic "i don't know" attitude concerning rebirth, samsara, nirvana or rather reinterpret them [as metaphors] than to greedily chase liberation which is just another craving and could foster laziness, selfishness and everything which is contrary to the teachings.
I totally agree with you here. There is a saying in Buddhism (paraphrasing): "Dharma must be practiced according to Dharma. When Dharma is practiced not by Dharma, it becomes anti-dharma". So, absolutely, craving for Nirvana, studying texts, trying to be pure and perfect, considering oneself spiritually superior to others, rejecting all the worldly stuff - can make one a horrible person and lead to a lot of suffering for oneself and others.
Dharma is subtle enough that it is but too easy to get confused and make it into an even larger problem than we had in Samsara to begin with. Which is why new schools were born, starting as far back as 200CE if not earlier - because advanced Buddhists who saw widespread confusion with dharma in their own communities kept looking for new and creative ways to teach real dharma without falling in the same old pitfalls. Hence the birth of Mahayana, Ch'an, East Mountain School, Soto Zen, Tantra, Madhyamaka, Mahamudra, and modern innovations of Buddhadasa, Thích Nhất Hạnh, and Chogyam Trungpa Rinpoche.
All of this is to say that your concerns and doubts are not new. Discussing what's wrong with samsara that needs fixing; problems with attachment to idea of liberation; selfish vs. altruistic motivation; getting lost in overthinking etc. - all these topics were part of Buddhism for 2600+ years - and luckily for us most of these questions already have answers.