Was the real Shadwell actually deserving of Dryden's scorn?
In a word, no. Certainly Shadwell was caught up in the humors tradition of Jonson, which Dryden decried, and one cannot successfully argue Shadwell was better than Dryden or even his equal. Dryden felt like perhaps Shadwell had sacrificed some of his art due to outside pressures, and the two differed on political matters. However, Shadwell was a talented writer; his comedies of humors were and are well-regarded by critics. Dryden himself doesn't seem to believe the worst of the man, for Mac Flecknoe's tone is rather affectionate, and the way he insults Shadwell is through irony, metaphor, and satire - not through harsh insults.
How does the poem take on the style of the Roman jeremiad?
Critic Margery Kingsley connects the origins of the satire to the Roman jeremiad, permeating his poem with prophecy, fate, and the relationship between the word and its realization. Flecknoe is a John the Baptist figure, ironically prophesying the coming of his dull-witted son and the end of all good poetry. Prophetic text was intended to remake the social order, and that is just what Flecknoe does: instead of "castigating or at least lamenting" the rise of Shadwell, he blesses him and "ultimately dictates the excremental future of his kingdom... and dooms [himself] to the underworld/privy and the 'Hell' of the Restoration stage."
What is a "poetaster," and what is Dryden's opinion of them as evidenced in the poem?
The definition of a poetaster, according to the Oxford Living Dictionary, is a "person who writes inferior poetry." It was coined in Latin by Erasmus in 1521, then used by Ben Jonson for the first time in English in his work Cynthia's Revels. Dryden himself was attacked with that derogatory label by George Villiers, though he did not deserve it. Dryden was happy enough to throw his fellow poets under the bus, though, if their inferior works were more popular or, more significantly, if they contributed to an overall decline of English letters. Dryden not only characterizes Flecknoe and Shadwell as poetasters, but also mentions the following inferior poets/singers/playwrights: Shirley, Singleton, Panton, Ogleby, and Etherege. This is Dryden's version of Dante's Inferno, for he casts his "enemies" into the "hell" of his famous poem, never to emerge from his scathing wit.