You said, "My question is the true Metta, which is a kind of goodwill to all without preference, without exception"
Metta is an attitude, right? Whether or not you actually can help everyone or not, whether or not you have Wisdom and Power to take care of everyone or not, whether or not you spend every minute of your waking life helping others, whether or not you try to help every person you interact with every time you have a chance -- metta is when you operate with an attitude of unbounded goodwill to all.
Which basically means, you try NOT to act out of ego and you DO try to act in "the best way possible" in every given situation. Acting out of ego includes mental, verbal, and physical action. Mental is the most important, it pertains to thinking any thoughts that are meant to feed the ego.
I'm pretty sure I know this topic rather well, because this was my "main practice" when I entered Buddhism. (Main practice is a feature of Tibetan Buddhism; it is something that for a period of time serves as a conceptual center of your understanding of Buddhism, the object of mindfulness, and the goal of all activities. Main practice can change as you progress from level to level.) You said, "Is it possible to exercise Metta to all?" -- I know it is possible because I have done it.
You said, "to all without preference, without exception" - not just "the beloved" -- yes, of course. This means you try not to have attachments, biases, overgeneralizations, preconceptions, stereotypes, any kind of identification, you always try to operate from an objective perspective.
You said, "How does one wish goodwill to all in this world where nature itself works by compromise, where one's goodwill is another's misfortune?" -- again, this means trying to be objective and unbiased, which usually means trying to find the best balance or the best compromise between all the opposing forces and interests. Once again, it's not about your skill, it's about your attitude. Whether you can or not is beside the point, the important thing is that you try.
You said, "how does one act to remain dispassionate?". See my answer about Kshanti-paramita. It's all about attachments. Ego is made of attachments. No ego - no attachments. No attachments - no ego.
Metta is one of the "Four Brahmaviharas". The reason they are called Brahma-vihara (Brahma's Disposition) is because they are supposed to emulate the attitude of Brahman, the Absolute God, whose body is pantheistically equivalent with this universe. According to Buddha, emulating the attitude of Brahman is a simple way to get pretty close to the perspective of Buddha, which then goes beyond pantheistic non-dualism of atman=brahman and into the triple emptiness of prajna-paramita.
So you basically operate as if you had no ego and were God, except you don't think that you are God, nor allow yourself to get a kick out of being so saintly and humble, since both of these thoughts would be the exact egoistic thoughts feeding the ego that you are supposed to eliminate.
