what we call rabbinic ordination today is a legal fiction. according to Torah law there is no such thing as ordination any more.It has to be rav from rav until Mount Sinai. This the Talmud says explicitly. And there is no disagreement with this in the Talmud or any later authorities. Rabbi like to hide this fact but it is none the less true.
So why do we accept converts today? We depend on Tosphot that we are doing the messenger-ship of the people that did have true ordination thousands of years ago. So far so good. But as for converts today it would seem if we are doing messenger-ship then it has to be done exactly like the rishonim said or it is nullified..