Your post shows you know the need to have a level shelf. However your results show the shelf was not level. You need to check levels on the “vertical” and “horziontal” as well as the two diagonals.
You also need to consider whether your pot was centred over the dammed space. If it wasn’t then you will have more glass on one side than the other, unless you had a very long soak at the melting temperature – ca 950C.
If your disc is 13 mm at one side and 6 mm or 10 mm at the other you have a more complicated reheating problem than a simple 10 mm or 13 mm all over disc. The thin side will heat through more quickly than the thick side. In any case the centre of the disc will be thicker than the sides. So you need to slow the initial rate of advance beyond that for even 13 mm. I suggest adding the difference of the thicknesses to the thickest part to get an indication of the schedule you should run. In this case 6 form 13 is 7, which added to 13 gives a schedule for 20mm glass. This may seem conservative, but if you are trying to save a piece conservative is the way to proceed.
Secondly, to get the glass to flow easily you will need high temperatures again. This may begin to give you problems with compatibility. So my suggestion is to use full fuse temperature only, but soak for several hours, maybe up to 4. This will give you problems with devitrification, so a borax spray or other devit spray is advisable.
This is not a simple problem, but if you are successful it it will prove very satisfying.