Anusim, T'shuba, and Halakha
In centuries past an anusi was not asked to give pedigree back to Sarah. R' Shlomo ben Shimon Duran in his 89th responsa requires absolutely nothing of a returning anusi. This opinion generally held sway for Iberian anusim returning to the Sepharade community.
Sepharade communities were overwhelmingly sympathetic to their relatives, friends and colleagues who were fortunate enough to able to make a return to the faith. The presumption, whether true or false in actuality, is that anusim are an endogamous community. General opinion held that an anusi was a Jew even if the anusi in question is NOT seeking t'shuba.
An unfortunate happenstance of our time period has caused certain populations to undergo background checks back to Sarah for Indians and Ethiopians, communities that not only are not anusim but have retained Jewish identity and ritual intact for centuries and are pointed out as Jews by the surrounding non-Jewish nationals of their diaspora.
In these cases the background check was clearly a ruse to deny these people their identity and rights in the state of Israel.
The Franco-German school from Rashi had a much stricter view on identity of anusim. The attitude is much sterner toward issues of identity and inclusion. They'd rather see a Jew die than even pretend to apostacy in life threatening conditions.
This attitude developed from circumstances that were quite different from that where the Inquisition was rooting out and destroying "Judaizers". Thus the halakhic response would of course be different.
Halakha is not decided without considering the time and clime. That is why halakhoth do vary between Ashkenazim and Sepharadim. Halakha is not an all covering blanket but a tailored garment that fits the particular case requiring a ruling.
Anusim would best be served by applying for return, if at all posible or known, directly to the community from which they were forced. Such a community remains in the know and knows exactly how to integrate its "missing" members back in the fold.
Likewise, isolated Hebrew communities whose links to the broad mass of Jewry have been severed, should seek to make ties with the communities they branched off of.
The alternative is the status quo — to be treated as a non-Jew and undergo full formal conversion. In some instances this will be the advisable route.