Is merely uttering the two Shahadas and disavowing every religion that contradicts Islam sufficient for a person to enter Islam after an explicit act of apostasy, and is he considered a Muslim if he does not repent of the action that expelled him from Islam?

If an apostate repents between himself and God, it suffices for him to utter the two testimonies of faith (Shahadatayn) along with regret and a firm intention not to return to apostasy. As for judicial matters, if his apostasy is proven, the judge demands that he declare his return [to Islam].

So, for someone whose apostasy was a departure from Islam, uttering the two testimonies of faith is sufficient.

However, for someone whose apostasy was due to a belief, statement, or action that contradicts Islam (such as denying an obligatory act or uttering insults), he must repent from that specific act of disbelief after uttering the two testimonies.

It is not sufficient to say, "I am innocent of every religion that contradicts Islam," if his apostasy was due to denial; because he might believe that what he holds is Islam. Rather, he must explicitly declare his return from that on account of which we ruled him to be a disbeliever.