Your question strikes me as confusing if these parts are supposed to be in agreement, because the first part implies uniqueness, solitude, or singularity - call it what you want - but the latter part implies multitude:
What do you call the person in a group whose opinion everyone agrees with?
... since opinions are offered separately and secretly, it's not true that other people follow this person's opinion -- they just happen to agree.
It's not obvious in which way timeliness is supposed to make a difference, or how you would know that who's first, if it's secret--unless you were either the omni-potent narrator, representing personal pseudo omni-potent mind state, or faking omni potence to effect the reader.
I might be mistaken and the latter part just describes "trendsetter". In that case, I beg to differ with the description.
If we can tell mind-state from action (cue behavioralism) then I'd recommend
as in first mover advantage; this is however not unique to agreeable positions, but often risky speculation.
If you aim to describe a point of a homogenous mass to describe it's inertia, impuls or momentum, I suggest
center of gravity
as a rather colorful metaphor. Also cp "apex", "turning point", "anchor".
For a meronymic abstraction I'd suggest
vice versa, for a meronymic pars pro toto--I guess that's what you are really looking for--try center of gravity on for size, again, equivalently try
model [citizen, voter, politician, student]
This fits e.g. the model student, who is like others of the same stereotype, and certainly influenced by prior examples, but by definition able to form an individual opinion. Ironically, they are often voted represantative (node to @JasonBassford). This is difficult of course, because the definition is circular, but at least it's not a contradiction. Similarly cf primus.
In the sense leader, but without winning over others, nor winning-over others, try ray of hope, fountain head, spear head, proginator, predator. For someone who's in contest with the others, alpha [male] obviously fits (incidentally not the center of gravity of a flock; that would be the females; the metaphor is lacking). I'm affraid I know no term that would be completely devoid of value judgement. If alpha followed its proginator, Semitic alef (seemingly "bull"), then it could have been semantically close to "bellwether".
Since your own answer, bellwether--which surely is often confused with the unchangeable weather and thus only loosely associated with the animalic metaphor--contradicts your second constraint, I will likewise suggest one:
not propagandist but "one who propagates [information, signals]". A propagator is in principle not the initiator, so it fits the bill.
After all, It would be really helpful if you added an example sentence.