Doitsu — thank you, that's exactly the evidence I asked for, and it settles it: no divergence. EPUBCheck flags big and center as RSC-005 errors, so it rejects obsolete elements just like epubveri does. We quietly agree, and there's nothing to file upstream.
The one thing worth noting is why your output shows two errors and epubveri shows four. EPUBCheck reports big (in the first <p>) and center (directly in <body>) — but not the font and strike, because both sit inside the <center> it already rejected, and the RELAX NG validator doesn't descend into an element it has thrown out. So its schema does exclude all five (big, center, font, strike, tt) exactly as I read it — the two that surface are just the ones not masked by a rejected parent. epubveri validates the children on their own merits too, so it names font and strike in the same pass. Same verdict, same IDs, same severity — one extra pass of detail, so you fix everything at once instead of re-running after unwrapping the center.
(And s / u stay unflagged in both, since they're still conforming.) Appreciate you building the test book — settling this on evidence rather than my guess is worth a lot.