Oh, there's other questions and I forgot.
Leather cord is... okay? I'd consider it specialty use. It's fine for drawstrings. It's fine for lacing, but that depends on the combined strength of a large number of laces. On it's own, leather cordage is pretty weak. Note that artificial sinew is usually just unwound nylon thread.
The strength of other thread is such: cotton thread<linen<generic polycord<maine thread<tiger thread.
To keep things from fraying, hit the end with a lighter or heated tool to melt it.
Double stitch isn't bad, but it's usually basically decorative. Used for machine run stitches to add strength. With a hand-run saddle stitch it's over kill, and when you stitch something by hand, no one will be able to pay you enough to stitch it twice.
Astorstat seems to be mostly an automotive product? Weird. In any case, for wet weather items, treating the leather with beeswax is great. SOP is to melt beeswax into either linseed or neatsfoot oil, and apply it to the leather. Use a heat gun once it sets up and buff it into the grain, then wipe off the excess.
Avoid petroleum product on leather if you can help it, they are typically not helpful for long term durability and might break down the grain.
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