“A conversion for the sake of marriage is completely pasul and even though some Batei Din allow it sometimes this does not mean it is ok. It would be the same as the rabbonim going into the house of every freie yid who eats chometz on pesach to replace the chometz with matzos. ”
Please easy with the am haaratzos
firstly such a geiros is NOT passul (though it might be bedieved) see Mechaber 368:12
http://hebrewbooks.org/pdfpager.aspx?req=9147&st=&pgnum=194
Rav Uziel has a teshuva on this in which he brings a teshuvas Harambam that essentially allows this mipnei takanos Hashavim
Secondly. Of course Rabbonim should prevent a frei Yid from eating chometz if they can. You comparison is a good one, but strangely it led you to the wrong conclusion
“We are better off without the people who will still intermarry after all the warnings and education against intermarriage, ”
Who gave yo upermission t opush away members of Klal yisroel. Yisroel af al pi shechatha Yisroel hu. If we can get him to wear tefilin great! Get rid of his chometz, of course we shoudl (in spite of you strange assertion) get him not to intermarry, again of course we should.
but if that sint an option of course he shoudl keep other mitzvos and please leave any evaluation of potetial geirus to competent poskim