the entrance doesnt even face the town!
It is a traffic source, not a traffic sink. The entrance/exit is on the correct side for the only thing that people do travel a long way for - Bletchley Park. Bletchley town centre is a run-down set of charity shops and takeaways and of no interest as a destination. Meanwhile, the vast majority of the housing that forms its commuter hinterland is on the side of the present entrance, and mostly arrives by car, bike or on foot because buses aren't timed for commuters, though the 4 does run via the frontage. If you observe where people go when they leave it, almost all the people that don't get in a car or taxi go up the Redway towards West Bletchley or turn right towards Melrose, with a smaller number heading down towards the bridge and the centre.
For the stadium you're better off walking up the road on the current side and over the "dodgy bridge" rather than via Saxon St. I *think* it's a bit shorter that way.
I'm not opposed to adding an entrance on the other side, I think it'd be a very good idea and am disappointed the East West Rail doesn't take the opportunity to do it. But if you had to pick one side, the current one is absolutely correct when you consider how the station is and is likely to be used unless there's a very major development of the centre.
Obviously Fenny loses out a bit here, but I don't doubt that a lot of people who go by rail from Fenny just pop onto the A5 by car and use MKC for its superior service, they can reach it in under 10 minutes, quite possibly about the same as going to Bletchley as the former is 70 most of the way and the latter 30 all the way.
Why is there no waiting room on the northbound platform?
Is the main passenger traffic demand (at Bletchley) southbound towards Euston?
Yes, very much so. Bletchleyites (!) don't use it to go to the city centre, other than rail enthusiasts, and Birmingham is a much smaller destination from MK. The mentality in MK is that internal travel is by car, or if you don't have one then by bus or bike. Trains are for going to London or secondarily Birmingham. Part of the reason for this is that if you're going to the shopping centre it's over a kilometre's walk uphill from the station, which eats any time saving from the speed of rail, though I'll give you that it's more useful if you work at somewhere like Network Rail or the new Santander building.