I should answer your query in nutshell and this will probably cover the points as asked by you:
EOW is basically a unit of crime branch that deals with the economic offences in short it is a specialized wing. Therefore, the procedure will be sameas that of filing FIR and as such there is no need of a lawyer to file a case before EOW, it is however suggested that each of the complainant files a complaint and that a complaint is not filed in group.
Yes it will be benificial for you to fight against the builder and this FIR will not materially effect your pending cases.
Regards