Is it possible to exercise Metta to all or is Metta only a mental state only? If so how does one act to remain dispassionate? Love, I mean the common one which cares only to the beloved and is indifferent to others, is accessible to many, but as I understand it this is the near enemy of Metta, not truly Metta. My question is the true Metta, which is a kind of goodwill to all without preference, without exception.

How does one wish goodwill to all in this world where nature itself works by compromise, where one's goodwill is another's misfortune? To give crude examples a lion eating am unborn calf or you frying unborn chick to feed you loving child.

I know the idea of Metta is not limited to Buddhism Laozi also has it in his gradation of the world we live in, he set the lover below the saint who champion a kind of universal love, which is Metta, but Laozi higher goal is even to abandon Metta and let go of everything to run as is...in a sense a lion eating a calf is part of the process of Tao so not evil. 

How's Metta seen in the ultimate sense in Buddhism, and how does one practice goodwill for the entire cosmos and
cultivate a limitless heart in everyday life?
"Practise yourself, for heavens sake in little things, thence proceed to greater". Epictetus (Roman Stoic Philosopher)

You have to use wisdom in order to make metta stronger and you have to practise metta formaly and informaly.

With wisdom I mean perceptions you have to reflect upon many many many times:

• No one can escape aging, sickness, death as well as mental and physical suffering

• Everyone is heir of his/her thoughts, actions and emotions. If one acts badly it's because of the causes and conditions that are meeting then and there