Is there any way to take a cruise ship to Hawaii, vacation on island for a week, and then take a ship home?

Yes you can do that at certain times of the year when many of the cruise lines have their ships doing Hawaii cruises rather than Alaska cruises. You would likely have to take one cruise line to get to Hawaii and then take a different cruise line to get back to the west coast, AND your return may or may not be to the same port that you departed from.

Probably not. Foreign flagged cruise lines (most are) traveling between 2 different US ports must stop at a far foreign country. That eliminates Canada and Mexico. There are round trips but you would not be allowed to take a partial trip for that reason.

There are one way cruises from Mexico or Canada to Hawaii but they are repositioning cruises and the timing would probably not work out.

Yes, Pay for 2 cruises.

Why don't you just take a load of Xanax and get over your fear of flying?

yes and no. finding two one way cruises that line up will be hard.