Putting the pipes on the outside of your 'thermal area' is a bad idea - you'll have much higher loses. If you have a good thick piece of foam between the water and the wall, then you'll be better off. I have no idea what you mean by ABS pipes (Acrylonitrile butadiene styrene is a kind of impact resistant rubber AFAIK). PVC or PE are the only pipes we have here. I would avoid trying to use the ground as thermal storage unless you can build a suitable insulating skirt around the whole house. The rate of transfer of heat is proportional to the difference in temperature times the contact area. Heat won't pass from the cooler to the hotter (you can try it if you like, but you far better nota...).

Why would you need to widen the opening to make it a Rumford design? It could be 24" to 48" wide. There is no "correct" height but typically a 42" wide Rumford firebox would have a height of 38" to 42" and a depth of only 15". The Jim Buckley website I gave you in another thread gives that information and other dimensions from his perspective. If you are gong to use Rumford proportions be sure you are using a mason who has done it many times before. If that person doesn't know the proper height of the firebox find someone who does rather than asking for this information from strangers on the internet; you need a good mason. I use the Masonry Fireplace and Chimney Handbook from the Masonry Institute of America as a guide. Any good mason should own it. It has recommendations for a Rumford and a modified Rumford fireplace. We have learned a little bit since 1796. The Rumford fireplace is about radiation not appearance. If you don't intend to heat your house with this fireplace and you don't intend the fireplace to be a historic replica, you don't need to follow all of Rumford's ideas. Here is a link that might be useful: Masonry Fireplace and Chimney Handbook

My condolences. Although this is a difficult subject, you have a wonderful writing style that just makes a reader want to read more. I look forward to reading more of your thoughts and life experiences. Regards, --John

found an interesting article about whether it's ethical to photoshop/alter photos in real estate... food for thoughts. Here is a link that might be useful: Altered photos: Removing the real from real estate