Who can legally access information from the government about my income tax returns?

Answer 1

You can. If you filed a joint return, your spouse can. IRS agents can, but only if their job requires that they look at YOUR information - they can and will be fired if they divulge the info to anyone, or go browsing through returns they don't have to look at to do their jobs.

Beyond that, there are situations where courts can demand your tax information.