Just looking at the photos I would say that the chain is Chinese made, it looks (and has come apart) very much like it. The Chinese never seem to get any weld penetration especially in the stainless so you’re only hanging off a small bit of weld around the outside of the link. Using Chinese made for anything important is silly and, as you can see, dangerous.
Also using stainless on a permanent mooring is a big no-no.
It is a long link as well, which is the weakest of the differing chain formations i.e. short link and etc.
Using 10 or 12mm on a permanent mooring would also be regarded as quite lite.
You basically had the nearly worst possible chain set-up on your mooring. Don’t stress, you’re not the first to do this and I suspect far from the last. Use big self-colour (not galvanised unless it goes in and out of the water
i.e. from buoy to boat) steel chain. Galvanised chain on a mooring can actually make it last less than self-colour sometimes, strange but true.
The shackle – A small stainless one a no-no. Again use a bloody big steel one, biggest that will fit the chain is your target and if it is bigger than the chain, so be it and very good.
As for stainless on an anchoring
system it is perfectly fine as long as you make sure of a few things:
1, It’s not Chinese made
2, It has been made and tested by a reputable company
3, You get whatever ‘grade’ you need to get the strength you want, just like steel chains.
4, You have a big wallet
There is stainless anchor chain available and made in the EU that is stronger than most steel chains being used. It is fantastic for anchoring
but the downside is no. 4 above.