Amnesty laws are laws that reduce the penalty of or allow the release of prisoners convicted of various offences i.e someone is convicted of an offence with a mandatory minimum imprisonment without parole possibilities, so can the US Congress in this case make a law that reduces the penalty of or pardons the person convicted of such an offence?
can the US Congress in this case make a law that reduces the penalty [retroactively]
The answer is yes. Even with respect to [minimum] prison terms, which it seems is what motivated your question, although "amnesty" has a broader meaning in the US being used even for immigration, or some rights [to be elected, etc.] as in the post-civil war "Amnesty Act".)
While there was a fairly high level of confusion on the 2010 Fair Sentencing Act, this was mainly due to the way that law was phrased, i.e. it was unclear how retrospective it was meant to be. The 2018 First Step Act was rather more clear, but albeit not on all sentences in its scope. However, in neither of these cases was the law challenged as unconstitutional, rather the debates and lawsuits were about what was covered by the wording. The discussion on these laws is somewhat complicated by the fact that Congress delegated specific sentence reductions to a commission. Said commission even published some stats in Aug 2020:
Retroactively Applying the Fair Sentencing Act of 2010
Since authorized by the First Step Act, 2,387 offenders received a reduction in sentence as a result of retroactive application of the Fair Sentencing Act of 2010. Offenders’ sentences were reduced, on average, by 71 months, from 258 months to 187 months.
There is a bit more discussion on Law.Se why such laws are constitutional in the US. It's essentially because they are not explicitly prohibited, despite the fact that Congress is constitutionally prohibited from legislating some ex-post-facto laws on some other matters (like making something illegal after the fact, or increasing the punishment for someone already convicted/sentenced).
The Amnesty Act of 1872 specifically removes the disabilities imposed upon most former Confederates by the 14th amendment.
Granted, this was a built-in feature of said amendment, not a more general power of Congress. Section 3 of the Fourteenth Amendment prohibits the election or appointment to any federal or state office of any person who had held any of certain offices and then engaged in insurrection, rebellion, or treason. The section also stipulates that a two-thirds vote by each House of the Congress could override this limitation. And that's how we got the Amnesty Act.
Interestingly, the Act is a bit ambiguous and can be read to assert that these disabilities are removed permanently for everyone past and present, regardless of connection to the Civil War, except for those exceptions mentioned in the Act itself. The first time anyone tried to assert this in a court seems to have been quite recently, having been brought up in an attempt to disqualify Madison Cawthorn from (re)running for Congress. While the court first ruled that the Act had removed all such disabilities for all past, present, and future congressmen (unless some other law changes the situation for the future), an appeals court unanimously reversed that decision. Another appeals court also ruled it did not have any affect on post-enactment incidents and people, and an appeal of that holding was dismissed as the court found the appellant didn't have standing, as they had not violated Section 3 of the 14th amendment in the first place.