The sylphs homeland is Moonspore Grove in the East Shroud, which by the time of ARR is overrun with the 'touched' sylphs corrupted by Ramuh (read: tempered), but in 1.0 Ramuh had not been summoned at that time (in fact, the sylphs stated then that they would not summon him - naturally this resulted in a schism in sylvaan society later on between those who wanted to adhere to the 'no summoning' policy and those that wanted to get revenge on the 'metal ones' (the Garleans, who stormed Moonspore Grove in a misguided attempt to eliminate them) - it should be no surprise that after the Calamity the later group summoned Ramuh and were then tempered by him, turning their kin purple and driving those who still refused to follow Ramuh out of their homeland, taking sanctuary in Nine Ivies, a location they name 'Little Solace'. It should be also noted that in 1.0 when the player was tasked along with their Path Companion by Minfilia to protect the sylphs from the Garleans, they had trouble just finding Moonspore Grove's location, so well hidden was it (the Path Companion pulled strings with a friend to find it's location out). Either way, for many centuries it seems the sylphs mostly kept themselves generally hidden and out of sight of most Eorzeans, like the moogles - although Gridanians even in 1.0 made pains to mention that they were perhaps the most benign of the beastmen, at least before they summoned Ramuh of course.
The Velodyna River is actually not yet accessible, but it can be seen by carefully examining the Eorzean map (it's the large river to the east of Gridania that forks into two, as seen here):
Well, it seems woodsin is one of those things that seems to build up over time - 1.0 showed Khrimm committing random acts of vandalism in the forest, cutting trees and setting them on fire and the like, and apart from a moogle stating that "this tree's elemental is furious!" it was only when he pushed his luck a little too far by setting a Hedgetree on fire did an elemental finally step in and punish him, turning him into a literal statue (unfortunately for Gridania the elemental went berserk and attacked everyone in the vicinity, requiring the combined might of E-Sumi Yan, O-App Pesi, Bowlord Lewin, Swyentha, Yda and Papalymo (along with the player, who was relegated to taking a back seat) to take the elemental down. O-App states that the other elementals forgave Gridania for what happened afterwards, but it outright terrified the Gridanians as to the level of violence that elemental showed. So basically woodsin doesn't cause an immediate reaction from the forest, but tends to build and fester, until a critical mass is reached by which point there is little anyone can do to save the person so affected - thus they're usually exiled as a wildling where the forest will deal with them in it's own way.
By the time of ARR though, it's actually stated during the Gridanian storyline that the Gridanians are actually noticing the increased incursions by poachers and the Ixal alike into the forest with the elementals apparently turning a blind eye, and it's downright scaring them - as you said such rampant desecration of the forest usually would not have been tolerated by the elementals and would have resulted in some kind of reaction. But nothing. It's generally believed that due to the aetheric changes caused by the Calamity, as well as the physical destruction wrought to the forest, the elementals have basically been weakened considerably - although they're like a wounded animal, unpredictable and still downright dangerous. The fact a low rank Ascian could enter the place and wreck havoc showed just how weak the elementals have become, resulting in pockets of elemental power like the Hedgetrees and the Guardian Tree Everschade being even more important than they once were. We still don't know exactly what makes the elementals tick - it's stated that as beings of pure aether they're virtually incomprehensible to mortal races thus their decisions or choices may not make sense to the mortals of Gridania, but the later are still bound to follow them anyway. It's the price they pay for Gridania's existence in the forest.