I;m not sure there is a blocking issue, since Dunbar and chathill have stopping loops, and these could easily be added at Alnmouth (behind the down siding). Berwick is more problematic, but use could be made of the loops north of the station, and even the bi-di signalling!
Reston already has siding on up & down which could be adapted to form stopping loops.
IIRC the current maximum passenger load on this line is currently 2 NX and 2 XC per hour, plus the northern DMu and the FSR 322 service at either end. IIRc there are 4 more movements per hour capable into EDB, 2 are reserved for the Borders Rail. One could be used for the EMU service to Newcastle, still leaving one which could allow NBE to go half hourly, or allow Newcraighall P&R to be every 20 minutes.
The line also sees a lot of freight - Infrastructure trains to Millerhill, DRS to Torness, the Edinburgh Bins, and coal to Alcan as well as the long distance stuff which passes over the full length of the line.
The stopping pattern would be difficult to judge, because to encourage commuting, you need fast times from Alnmouth & Widdrington, and yet the P&R market from Morpeth and Cramlington should also be exploited.