Before any EGIP started, there were 2tph Queen Street HL to Cumbernauld and points thereafter. After Springburn-Cumbernauld was electrified, that reduced to 1tph, with 2tph coming from the Springburn electric terminators being extended. Originally the plan was that post-EGIP, the 2tph Cumbernauld service would return, with electrification being the main change. Now, the plan is that the 2tph Springburn terminators will continue to operate in addition to these services, providing a good combination of frequency, speed and connectivity to Cumbernauld that makes everyone happy. The 2tph HL electric trains are those that will now run through to Edinburgh as a stopper.
The only real justification I can see for a Grangemouth station now is if extra trains run from Glasgow to Falkirk Grahamston but they cannot be accommodated east of there. If so, they'll have to turn back on the branch anyway. If the timetable is right, then the cost of a Grangemouth station would just be the signalling and the platform needed to run a passenger service. However, it may well be the case that the business case isn't that strong even with a low cost. The Grangemouth area in general isn't exactly badly served by rail, with the two Falkirk and Polmont stations being not too far away. If you're interested in the fastest possible journey to Glasgow (and Edinburgh) then a drive to Falkirk High for the E&G would be hard to beat. This is especially so if there's only a single train per hour at Grangemouth. Alloa's 1tph service works fine because there isn't really an alternative at peak times that doesn't involve driving along congested trunk routes. Realistically then I think you would need at least 2tph up the branch to make the platform worthwhile. Extending the 2tph Cumbernauld stopper would almost certainly be possible but then the journey time to Glasgow would be very long, making the branch even more unpopular compared to the E&G route. A simple shuttle bus with through-ticketing that runs to the two Falkirk stations might well be a better use of Transport Scotland funds.