Irrigation always builds up salt in the soil. Areas of California are falling out of production for this reason, after a good 100 year run. Irrigation is what turned much of the middle east into desert.
The rest of your points I guess boil down to needing infinite cheap energy. Anything's possible with massive amounts of free power, I suppose. I wouldn't bet on it.
Large areas of China, Egypt, ect have been under continuous irrigation for thousands of years and they can still grow just fine. If California is having issues with irrigation over what 100 years then it's a separate issue.
They probably do have issues due to the amount of rainfall vs. irrigation and their crop rotation etc. But, it's not really an issue with irrigation just their approach to irrigation. Desalination also gives you the option to have much lower salinity than river water which should allow areas like California to cope with this issue.
The rest of your points I guess boil down to needing infinite cheap energy. Anything's possible with massive amounts of free power, I suppose. I wouldn't bet on it.