Board Thread:Questions and Answers/@comment-36170398-20180721134224/@comment-28802873-20180722122447

Kevin didn't exactly absorb the DNA to give it to the Amalgam Kids, he served as an intermediary between them. He absorbed DNA that was immediately transmitted to the Amalgam kids, which is why he didn't mutate when we saw Alan get his powers. By doing that, Kevin continuously absorbed DNA without reaching his limits since what he absorbed immediately went to someone.

For Ultimate Kevin, I guess that, since the Amalgam Kids spent years with those abilities, training with them and simply growing with them, they became a part of them and were returned to them after that the whole Ultimate Kevin mess was over, which is why Pierce had his powers back: their powers were mixed with their human DNA, making it that they weren't human kids with alien powers and more like powerless hybrids