Ok I shall try to fill this in
1 It began around February 3rd 2525
2 250,000+ humans escaped
3 There were odsts but no Spartans unless you count sgt Johnson as he was from the original Spartan program
4 there was a cruiser in orbit but it was destroyed the only defences on ground were some unsc soldiers and local militia
5 No harvest was on the very edge of unsc space most ship yards are built in the inner colonies
6 I think that's it .