Hi there, welcome to Cake Central.
Its great you have some photos of the cake post-crash.
I believe your 12" tier supports were insufficient, and/or unevenly cut. There is every chance the table was shoved from the front causing the backward movement of the cake to occur, and hence initiate the collapse at the rear of the 12". But this is something that you CANNOT blame on anyone, because you do not have evidence.
For cakes this size, I would have had 8-10 dowels in the 12", 8 in the 10" and 6 in the 8". There is nothing wrong with using wooden dowels, so long as you cut them properly and level each tier as you go. I also would double up cake boards under the 10".
I know you are saying you will no longer use wooden dowels, but in case anyone else is wondering how to use wooden dowels for support while reading this:
1. Use a template on the top of the base tier to mark where you'll place your dowels.
2. The dowels should sit around 2cm from the edge of the tier above. Arrange the dowels markings so that they are equally spaced, and add a centre one also if desired.
3. Get down and eyeball at the level of the tier top to see if any part of the tier is slightly higher than anywhere else.
4. Using the pointy end, place your first dowel into the highest point of the tier (within your selected doweling points), ensuring it is going in at 90degrees. You can use your level for this (from front and from the side).
5. Using a pencil, mark your dowel about 3mm higher than the fondant skin.
6. Carefully unscrew out this dowel, wipe clean.
7. Use a sturdy flat border, such as the straight edge of your gumpaste board, and place the point of your dowel against it at 90 degrees, then align EACH dowel in turn and mark for cutting exactly in the same place as your first dowel. Do not try to do all at once along a row of dowels. Repeat for all the dowels.
8. Use sharp, secateurs that you reserve solely for this job. Carefully cut each dowel straight across the top. Check that each cut length is the same as the height of your marked pointy dowel.
9. If there are ANY discrepancies, use sandpaper or an emery board to get the perfect height across all dowels. If any are too short, discard and repeat step 7.
10. Clean your dowels before inserting into your cake tier - off any dust or pencil marks.
11. Insert each dowel into the cake tier, using your level to ensure they are going in perpendicular to the table surface. Remember you need to check 90 degrees from the front and 90 degrees from the side!
12. When every single dowel is firmly in the cake, check that they clear the tier surface by 3mm (approx), then place a flat board on top of all the dowels sticking up a bit, and check (front and side again) that the dowels are now all LEVEL.
OP - can you please explain what happened on the wedding day? Did you replace everything so they had a full 4-tier cake in the original design? Can you post a photo? Because if you provided a cake exactly the same and on time (before the reception) - no refund. If you provided cake but not to the specifications, I would issue a partial or full refund. If you didn't supply anything in time for the reception - full refund, and a free cake to a nominal sum.
I also have a policy of not accepting liability after my cakes have been set up or collected. This is signed for upon delivery/pickup and I take photos of every set up. Why did the cake get delivered the night before (just out of interest)? Seems risky.