It's unusual for you to suggest re-opening a line in Northamptonshire!

Which line are you thinking of and is it still intact?
I'm not - you clearly didn't read my post properly.....
Chiltern presently orients a key part of its service to being a through London-Birmingham budget-but-not-as-budget-as-LNR service. That will no longer be required, and it will be able to revert to being solely a regional commuter type operation, which will allow a timetable recast for more stopping services.
I'm not so sure - with the WCML there is a known problem of long-distance services which don't stop between Rugby and London taking up paths, that's not the case with Chiltern.
So whilst Chiltern do run a London - Birmingham service (2tph) it's worth pointing out that these all have multiple stops: Wycombe (1tph) Bicester (1tph), Banbury, Warwick / Parkway. It's as much about providing those places with a service to London and to Birmingham as it is about providing an end to end service.
If you start inserting stops you'll slow down those journeys - so let's say, for the sake of argument, you insert Haddenham and Princes Risborough into those services - try telling the good people of Banbury that their dividend from HS2 is that their journey to London will be 10 minutes slower. Or Wycombe passengers that their Birmingham journey will be 10 minutes longer.
The alternative is you break the service and don't run through Marylebone to Birmingham services - OK, so where do you break it ? Banbury ? Leamington ? So Bicester and Wycombe lose their direct link to Birmingham - and people go back to using the M40 instead.
HS2 doesn't offer Chiltern any real opportunity for capacity gain - the issue with Chiltern is it's a 2 track railway with relatively slow linespeeds, on the Aylesbury side interactions with LUL. Currently it's sending (off peak) 7 tph out from Marylebone, which is the same as Fenchurch St - another line with limited capacity.