Hi John,
You are trying to crack an age old conundrum: How to make a GWR BLT that is believable but a little bit different. You are definitely on the right track.
All permutations of positions for platform, goods shed, engine shed, cattle dock, etc, are possible so long as they can be operated reasonably sensibly.
Position2 for the cattle dock is very Moretonhampstead, as you know.
You are right not to bring the tracks too close to the west end of the boards. Even a little bit of non railway scenery there will help to set the station in the landscape.
The bay platform is a very useful modeller's cliche but in your period would a bay be likely? Amongst models, the lack of bay platform would be a factor that made yours more unusual! (Unlikely to have smelly livestock too near the passengers, BTW, although even that permutation was known. See cattle pens on the platform at Princetown!)
You always need some form of trapping between the goods and the passenger lines (to do things properly).
In your period its very likely that there would have been a small turntable (~45ft) at the station - possibly in the middle of the engine shed siding or in a kick back off it.
Consider making the design more dynamic by angling and/or curving the main running line. (I'm sure David didn't mean literally straight, just that the dogleg was undesirable.)
As David said, a long back siding or mileage siding for unloading open wagons would be common but wasn't always found as a dedicated length of track. See Ashburton.