The rules are exactly the same as for Year 7 allocations.
Children in care.
Children living in the catchment area of the school.
For Year 7 admission in September 2010 - Siblings of children in Years 7 to 10 who are on the roll of
the school at the time the allocations are made.
Children who have exceptional medical or social needs supported by written evidence from a doctor, social worker, educational welfare officer or other appropriate person.
Once the above rules have been applied, then any further places will be offered in distance order
Catchment children get priority over OoC, so in your list (2) gets priority over (1). A non-catchment Bucks child would be treated the same as an OoC child and the allocation made accordingly under the distance rule.
I am assuming that in item (3) you are referring to the situation where someone has listed a very popular GS - eg. AGS or AHS first, more in hope than expectation, but put another school as second preference, e.g. SHF? The same rules apply: provided the child lives close enough to the school the allocation would be SHF.
In my 12+ Sticky I mention this point. The first preference school can be the one of your dreams, even if there is almost no hope of getting in, but the second preference should always be your catchment school and also one that you can be reasonably hopeful of getting a place at.