You miss the point about double quoting, to include double quote in field you should double it:
"=""634576345" => ="634576345
To keep leading zeroes you should present your numbers to Excel like strings so they should not be converted to numbers at all. I have no Excel installed anywhere right now but here are possible solutions that come in mind:
- using comma instead of a point (or point instead of the comma if your locale uses comma as a decimal separator);
- adding leading space symbol;
- adding leading quote (') symbol (AFAIK Excel does this to distinguish ordinary strings from some-other-type-look-alike).