Dead Sea Salt- ditto what Obsidian and not_ally said.
Grinding salt for salt bars- also ditto what Obsidian and not_ally said. From what some here on the forum have experienced, grinding salt with a typical household grinder makes sharp shards out of the salt, and when put into salt soap, it can actually cut you when you rub the soap on your skin. The fine grind of salt that you can buy at the store has much more rounded edges. I would use that intead.
Regarding your formula- I'm going to sound like a broken record, but ditto what Obsidian said.
20% coconut is not nearly enough for a salt bar if you want any kind of decent lather out of it (although 'decent' can be subjective, of course). Salt inhibits lather, and the large amount that is used in salt bars can outright kill it completely....unless you use a goodly amount of coconut oil to balance it out (coconut oil soaps are one of the few soaps that will actually lather in salt water. Others are PKO and Babassu).
Re: castor: castor oil actually won't add any lather to a soap at all, but it will enhance the lather that is already present from other oils such as coconut, PKO or babassu. It gives them a certain 'oomph', if you will.
I agree with not_ally in regards to the argan. I would save that for leave-in products such as lotion, etc...
Re: when to add salt: I always add mine at the very last right before I pour into my mold- when my soap is at medium to med-thick trace. I should mention that I don't do fancy swirls with my salt bars. I color them one color up front, or I do a simple and quick ITP swirl before I pour. I add my FO's up front to my oils, by the way.
Re: coconut milk: I can't speak for others, but I add it to mine because it gives a lovely depth to the fluffy bubbles made by my 100% coconut oil. I mix my lye with water 50/50, and add my remaining water amount as coconut milk fortified with enough coconut milk powder to bring the milk concentration level up to 100% for my batch. This I add to my oils before the lye solution.
Use SoapCalc's default "full water" amount when making salt bars (or any kinds of bars) if you want to slow down trace. I personally wouldn't use more than a 'full water' amount, though. Too much water will give you warped bars during cure. And too much water can also cause separation in your batter.
I cut mine as soon as they firm back up from full gel- usually within 2 hours, more or less, after pour. The soap is still quite hot at this time, so I use gloves.
Not-ally, welcome back. I missed your banter
Me, too!
IrishLass