You had already answered that point.
The statement 'If used, guards must all be in the same mixed division' is still correct even though there is only 1 guard unit.
It does NOT say "ONLY" guard infantry. Therefore you can use other non-guards in the same division.
As you so rightly point out - the statment could quite easliy be left out.
PS. Any decission on the fix for the 1815 Prussian list?