It doesn't matter if the new platform tracks don't connect to tracks east oif Jamaica. Tracks on existing platforms will continue to connect to the Atlantic Branch, allowing trains from LI to go to Brooklyn. The Atlantic branch will not be "completely severed" from the rest of the network.
No particular need for tail tracks there. The leads to the 207th St. yards are just south of there, so tail tracks are not needed for train storage. They would help by allowing faster acceleration into the station, but many terminals don't have tail tracks.
The answer is no. There was never any connection or daylight between the subway tunnel and the trolley terminal. In addition I believe that by the time the subway got near 2nd Ave., it was well below the level of the trolley terminal.
Do you know how narrow the service road is? One driving lane plus one parking lane.
Hudson Rail Link has teeny tiny buses, not suitable for BRT. Big buses would have a real problem on the narrow, curvy, steep streets.
I know, I live a few blocks away.
Do you have any idea how narrow the HH Parkway ROW is? No way could any tracks fit there, even if elevated. Also, the route on your map to and from Riverdale MNRR station goes through what's called the "estate area." No way those residents would want rail on their streets. And the grades uphill from MNRR would also be impossible.