I know there is debate regarding alleged discrepancies in Lacher's testimony and his exact whereabouts during the execution, but from the Whites' point of view, if they knew he was a member of the guard and he was inside the house at the time of the murders, they probably wouldn't have examined the evidence against him with a magnifying glass. After all, in real life, Letyomin and Starkov didn't take part but they were still executed by the Whites.
I also read on wiki that when Arkhangelsky was chair of ROVS he favored a pro-German policy.