I built a rock wall as well about 8 months ago and it has been through everything. superworms burrowed into it, crickets made a home inside the cracked part of it and made it worse, the cubby I made for her broke off. But, yes you can repair it the same way you made it the first time. It will be much easier now though since the foundation is already there, all you will have to do is just apply more coats over it. if it was cracking the first time that is because you did not have the mixture right with the grout and yes you are supposed to seal the grout with mod podge. I also put a light layer of (spread about a pinch) sand on the cubby so she could climb it if she wanted (which she does all the time so I am glad I made that decision). If you are unhappy with it just take it out, put more coats of grout on it, and seal it with mod podge.