Re option 1, there are plenty of good primary schools in Chelmsford and if you relocate, the options for your DS are wider as there are plenty of GS such as KEGS,CRGS, Southend and Westcliff and there are some decent local state comprehensives. Chelmsford is a lovely place to live, but then I may be biased
commuting to Cambridge by car will take about an hour (A120 and M11) and you are going mostly against the traffic - unfortunately I am not aware of any decent train or bus option.
Re option 3, check with CSSE on local buses. I believe that ChCHS has most extracurricular activities during lunchtimes so participating in other activities will not be as much a logistics puzzle.
I think it boils down to which school your DD prefers and the amount of financial risk/family sacrifices that you wish to take now and for the next 10 or so years.