Only a guess, but with a Chippenham-Bristol shuttle running and laying over for 15 minutes an hour at Chippenham on platform 2 you might have congestion problems.

I don't know the answer on this one.  But thinking about it, if the train service ran at (roughly) the normal times, it would be a long layover at Chippenham ((but then 2 trains can reverse there at the same time)) ; if the train did an out-and-back from Westbury, you're potentially looking at a lot of shunting and re-platforming there.    And as noted there's quite a bit of extra traffic from the Westbury area to Bradford Junction today and tomorrow - unlike some other weekends, it's not just the London to Bristol trains, but London to South Wales too.    I would also venture to suggest that a lot of traffic on trains coming up from Westbury into Chippenham on a typical Saturday usually stays on the train to Swindon (much more than on a Monday to Friday) and with all these things together the balance was probably to a decision that it's best to do it with buses.  Not thrilled, but not an issue top of the "critical" list either.