Section 174 Cr.P.C. is applied whenever there is an unnatural death as is mentioned in that section. However, if there is a specific offence made out (such as murder under Section 302 IPC), there is no need to file a case under Section 174 Cr.P.C. Similarly, after filing a Section 174 Cr.P.C., if the police finds that there is a regular cognizable offence made out within the meaning of Section 154 Cr.P.C., it can immediately register FIR under the relevant sections. They need not wait for RDO enquiry for that purpose, since police has the staturoty power of registering FIR and conducting investigation. It appears that in this case, the police has registered offence under Section 304B and 498A of IPC on the basis of coming to know of the commission of a cognizable offence after first having registered Section 174 Cr.P.C. case at which stage it was only an unnatural death case.
Reply to your questions is as under:
(1) Charges are to be framed on the basis of the police investigation which cultimantes in the submission of a final report (such as charge sheet) as envisaged under Section 173 of Cr.P.C. on completition of investigation. RDO enquiry will be irrelevant in this regard.
(2) Since the police have the statutory power of conducting investigation one a cognizable offence is made out, they need not wait for RDO enquiry. Thus, it may not be possible to get an acquittal (actually, it would be called discharge) on that basis alone. Of course, you can apply for discharge on merits of the case if sufficient evidence is not available or if the offence itself is not made out.