I don't think it matters how long you mix as long as you at least mix long enough to dissolve the salt, or a better way to put it is you can't mix too long. The best rational I can give is that we never stop mixing the water once it's in our tanks.
In my case I mix salt in batches then let a portion sit in my saltwater container for up to two weeks. I haven't seen any ill effects. My container does have a lid to stop evap.
You can also absolutely run a automatic water change on a reef. I do so with no ill effects, and much more importantly, many much more successful reefers do so as well. David Saxby runs one and he is at least amongst the most if not the most successful reefer.
I am of the opinion that small water changes over time are of less shock the system than single larger changes.
I still use a salt that mixes to parameters that are very close to my target parameters. If you dose alk and cal your salt can even be over or under your tank, you will just need to adjust your dosing accordingly. With both happening all the time your not causing any swings in your parameters.
Any inbalance an automatic water change could cause can be detected with regular testing, in my opinion the same rate of testing that would be required if your are dosing alk and cal.
I don't heat the water, or keep it mixing, or anything. The amount that's being changed at any moment is so small compared to overall system it won't matter that the ph, oxygen levels, and temp will be off.
My assumption is that the constant change is adding back trace minerals that are depleted and removing those that are constantly rising. It may remove some nitrates and phosphates but I don't think it's a meaningful or effective strategy.
