WE HAVE A WINNER!!! Congratulations Laurelinarien! Please let me know your in-game name, so I can send you your 100 gold prize!
Now, to offer a full explanation of the riddle. I will start with the last lines, because as a tricksy hobbit I purposefully put the most obvious clues last, and the least obvious clues first!
Yet failing it seems am I
As the four with me do spy
This scarecrow is a complete failure. Not only is it not scaring away the birds, but there are 4 crows, or perhaps more accurately crebain, literally sitting on it. These birds are commonly used as spies by Saruman and perhaps others.
Quivering, yet un-bowed
Presenting a choice in the road
Looking at the scarecrow, we can see what appears to be a quiver hanging from its shoulder down to the ground. However, it has no bow (unless that stick is meant to be one, though it has no string). It is also presenting a type of fork - as a choice in a road is known - a pitchfork, to be exact.
Note also that every two lines rhyme. The hyphen in "un-bowed" differentiates it from "unbowed" (i.e. refusing to bow). And to make it rhyme with "road" it had to be pronounced "bowed", as in a bow and arrow. These two subtle elements were intended as clues that the "bow" might pair with the "quiver".
With a vessel am I crowned
Blind seers seated, round
Looking again at the scarecrow, we can see that in place of a helmet it has a bucket, which is a type of vessel for carrying water or milk. Looking closer we see that its eyes, or "seers" in riddle-speak, are seated upon its head (not carved like a statue) and they are both blind and round as they are buttons! Incidentally, the comma between seated and round indicates that round describes the blind seers, not that they were "seated 'round". This was purposefully written this way as a possible misdirection.
Mystery of Bree
Smoking breathlessly
Lastly, those that look very closely may see that this scarecrow has a pipe. And not only does it have a pipe, but that pipe is lit and the scarecrow is actively smoking it! How this is possible is clearly a mystery - how could a scarecrow, with no breath, be smoking a pipe!
Of course, this last/first clue was very subtle and likely not obvious to most people. It also served to make people think of chimneys or other things that smoke within the game. Again, this was a purposeful possible misdirection...
...and that's it! Thanks to all of you who played my little game and congrats again to Laurelinarien for being the first to offer the correct answer!