When the MTA LIRR's Eastside Access opens up, it will take off some of the pressures at NYP and it might even be a lot, in some cases. This will hopefully get Metro North a lot closer to running select New Haven Line trains by way of the Hellgate Line into NYP, a much needed project for years.
Yes, St. Albans is indeed considered the Montauk Line but while most of the trains running on that stretch are going as far as Babylon, many continue east of Babylon. Obviously the Babylon Line is straight in so many areas with no grade crossings, enabling a 79 mph operation. I'm not sure how many interlockings there are on the Babylon but I can imagine that there are many, more than on the Main Line and Central Branch.
Running any train heading to the Montauk Branch by way of the Main/Central poses some issues-yes, getting stuck behind another train, especially a local. After Queens Interlocking in Queens Village, the next interlocking east, I believe is Nassau and after Nassau, it's Divide. When the third track opens up, I wouldn't be surprise if there is an interlocking installed at the east end of Floral Park station since probably one of the tracks would diverge from the Hempstead Line. Presently having two tracks on the Main Line limits increase in service to the Central Branch.