It does look as though a southbound through from P4 would need to move across to the down main at Oxford North, which might not be ideal (though down trains from 4/5 mostly seem to use the relief in my experience, so it may work. The Oxford North crossovers are high speed at least, and the others are close enough to the platforms that stopping trains won't be moving fast.
But ideally you would want to keep the Cowley to Parkway trains on the eastern tracks to avoid two crossings of the Cherwell/ Cotswold to Didcot Up main.
Ideally yes, although as all passenger trains stop at Oxford, the ex-Cowley train would only need to clear Kennington South Jn as the train from Platform 3 is departing, and equally would be able to have a slightly longer time in the platform if the following train from Wolvercote needed to pass ahead of it before departing towards Bicester.
Freight is arguably a bigger problem although much of it is booked to stand in the Oxford/Hinksey area anyway. And if the platform needs to be vacated you can hold an ex-Cowley on the middle road at Oxford North Jn while the other trains run on the outer line (I can't remember if the Relief and Main are switched here to reflect the preference for Northbound freight to be held on the inner line.
Another problem here is that southbound trains often use the Up Relief to Hinksey North Jn to allow freight to be pulled up clear of the north end of Oxford station. Which means the 10 minutes for a wrong-road moved thorugh 3 will be more like 13-15.
(It's hard to talk in terms of up and down trains since you go both north and south to get to London...).
Yes, the Up Chiltern Trains run in the same direction as the Down EWR between Oxford and Bicester, it's all very railway!
