I'm not great with r-codes and zoning, but I don't think you'll be allowed more than 2 houses on that size lot for r25...
R25 requires avg 350sqm, 3x350=1050, 1050>921
Yes, They would. But you'll also need this as part of your subdivision, prior to building stage (as well). WAPC wants earthworks plans, drainage etc. (others can give you more specific information about this)