I don't think Lockjaw being an Inhuman is inconsistent with what came before. The Fantastic Four called him a dog (usually "Crystal's dog"), but the rest of the Royal family didn't. I'd also argue that the issue of The Thing where he did that told a very interesting story. It wasn't changing continuity for the sake of changing it but because he thought it would add to the mythos and make for a good story. Likewise, his involvement in resurrecting Jean just feels like work for hire. To my knowledge, he wasn't involved in the plotting of the resurrection device itself.
But there is one good example of his reactions with the X-Men line that suggested the sort of pettiness I felt in his West Coast Avengers run. It's not a retcon, though. The whole Trial of Galactus storyline was a spat with Claremont. He had Reed save Galactus. Claremont was upset because he had to kill Jean for killing one planet but Reed gets to save Galactus who has destroyed thousands of planets. So Claremont put a scene in one of his stories where Lilandra tells Mr. Fantastic that, if Galactus kills again, the deaths would be on their hands. So Byrne responds by having Galactus destroy the Skrull homeworld and Reed Richards get put on trial. The funny thing is, from a reader's perspective, it's one of the best examples of cross-continuity of the era. But it was really just a pissing match between two creators.