Prince and Princess Michael rarely carry out official engagements, so it's not abnormal for what they do to not be mentioned. Michael might attend the reburial, but, unless something comes out of BP naming him as EIIR's official representative, he's attending in a private capacity as a friend of the Romanov family -- and private engagements aren't listed in the Court Circular (almost all of Michael's engagements fall under the category of private engagements).