Mary could, of course, try to game the system and find a nonce for her block (block i+1) which would cause the sharechain network to "choose" her as the assignee of the bitcoin reward for the next block (block i+2). This may be a problem in the beginning when the sharechain network difficulty is extremely low. However, as soon as the sharechain difficulty finds equilibrium this attack becomes expensive in that it will require significant extra work to pull off. That work is probably better spent simply mining according to consensus which often will mean assigning a shareholder other than onesself as the would-be-winner of the bitcoin block reward. Afterall, for every valid sharechain block Mary mines, she does get to assign herself the shares!