Thanks for the input on the northern portions of the trail! Weather permitting, I'm hoping to do a round trip well north of Elmsford tomorrow, as a stepping stone to then doing the ride back from Brewster.
In the meantime, here's a picture from last weekend of a stretch of graded dirt between an on-ramp and the west bank of the river which I hope is the southern end of the connector.
While looking for something else, I also happened on an article with details:
North Trailway + South Trailway = ? (annoying auto-play video)
It will cost Westchester taxpayers $2.8 million for the final connection, which includes a tunnel under Interstate 287 and the link to where the North County Trailway now ends on Warehouse Lane.
So it sounds like the not-yet-paved portion in this picture runs north, will go under the elevated 287 in it's own "tunnel" and then connect up with the portion along warehouse lane that I saw already paved (but still roped off) a few weeks back.
But it won't be perfect, requiring a detour on the south side of main street to the east to cross and then back even further west across the pictured bridge, to where they'll presumably be altering the railing to give access down to the trail.
Attempts to make a crosswalk at the trail head proved not possible. So cyclists will have to dismount, walk a half-block along the sidewalk, cross at the crosswalk at the light, and then walk on the north-side sidewalk to the new route.
Perhaps best combined with a water/snack stop...