If everything is stopping at Watford then you do not need loop platforms in the first place so the whole problem of how to fit them in does not arise. The whole point of loop platforms being, that they allow some trains to stop without delaying those following.
On the contrary, if everything is stopping at Watford you definitely need more than one platform per line for them to call in, or you severely reduce the number of trains you can fit through. Trains can currently run 3 minutes apart through Watford; but a stop will cost about another 3 minutes in journey time and force trains to be about 6 minutes apart. In other words you'd only fit half as many trains through!
The point of a loop platform, like at Milton Keynes, is for consecutive trains to be able to call without reducing the headway between trains. I believe that was the original idea behind putting in the extra platform at MK; shame it's not really used to that effect.
A better example might be platforms 4 and 5 at London Bridge: trains leave Charing Cross every two minutes in the evening peak, and then alternately call at platform 4 and 5 at London Bridge, which can cope with a train every 4 minutes but not more than that.
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That said, Again, if everything is going to be stopping at Watford, then slower geometry can be used that might fit.
Ah, but the beauty of Milton Keynes is that trains don't have to slow down any earlier to cross to platform 5, because the turnout is 75mph. That means trains can call in either platform 5 or 6 at MK with no difference in journey time.
If platform 5 were reinstated at Watford with a slow turnout, then you'd have to allow for that, and again you'd just end up reducing capacity through Watford compared to having fewer trains stop.