I didn't notice an exact duplicate of this question anywhere, but it's quite common for canned coconut milk to separate leaving an opaque white solid layer of fat and solids, and a thin watery/syrup layer of cloudy coconut water.
Coconut water is naturally a little tart, so I don't find that concerning. If the can is in date and showed no signs of swelling ...
They should be submerged. Lactofermentation is done anaerobically - under the brine. If it is in contact with air, mold starts to develop (mold is aerobic). Also yeast has an aerobic mode, where it multiplies, so the berries might become slightly alcoholic.
Either put brine on top of the next batch, or squish/bruise some of the berries.
If it's darkening that begins at the top, then I reckon it's oxidation.
Get air bubbles out first (vibration, tapping) then pick a technique to remove air or replace with nitrogen.
Freezing would also slow oxidation but not ideal for many textures.
The advice to use a 'canning salt' no doubt comes from a country (such as the USA or Canada) that adds iodine to table and 'ordinary' cooking salts.
This is not required or standard, if it even happens at all, in the UK; so you don't need to worry about it.
You may also want to avoid 'anti-caking agents' though, which are typically added to fine table salts, ...
My mother has a recipe that you have to mix in 2 to 3 days. The reason being as in a large quantity the centre can heat and rot. I just finished 17 large heads of cabbage in a 20 gallon crock. I mixed on day two. I could feel heat in the centre. I now leave till ready.