Soft Halyard or Line Shackle

[allene]

I would encourage you to use either a brummel or stitch, not a whipping.

If I understand what you are doing, I do not think the multiple diamond knots will hurt the strength of the assembly. The weak point will be the joint at the diamond knot and the eye of the line shackle.

These things typically break at the knot. You should have a strength of about 1/2 line strength but if your setup is doubled as Brion says, then getting about line strength would be right. It should not break at the splice so that is a huge mystery. You should have close to line strength there. Perhaps I am not understanding the setup.