It is absolute.
> Can a US president basically compose an edict saying 'This man is a bleeding hero and no branch of the US government will ever hold any of his actions related to incident X against him, now or in the future or else they shall snuff it!' and have this respected by his successors?
No, a pardon only affects criminal prosecutions and punishments -- to the extent covered by the text of the pardon, any future prosecution (or continuation of existing prosecution) is barred, and any criminal punishment (including and disabilities imposed by past conviction) is terminated. It doesn't affect any other action by the government, including any civil action that might be available in law.
> Is it narrowly defined, eg. the pardonee (is that even a word) may not be prosecuted for his violation of US code such-and-such, section so-so committed on or before date DDMMYY? (Leaving any successor free to throw the book at him for anything not explicitly covered by the issued pardon?)
It can be as narrow or broad as the President chooses. About the only limit is that it is generally held (though this has never had occasion to be tested) that the Constitutional power of the pardon does not extend to prospective pardons (that is, a pardon cannot apply to offenses that occur after the pardon is issued.)