Can an employer lower wages or stop payment totally in "at-will" employments ?

i. e in non-contract employments

I'm not sure what you mean by stopping payment. In all cases they must pay you for any work performed. The payment must meet the minimum wage requirements of your state, with the exception of jobs that are exempt from minimum wage (i.e. waitress, taxi driver, etc.)
An employer can lower your wages with advance notice providing the wage remains above minimum wage. They can not take this action retro-actively. In other words, they can tell you that you will being making less starting now, but they can't say that they will be paying you less for work you have already performed.

In the US of A, even an "at-will"/non-contract employer must abide by state & Federal minimum wage standards for work performed. The employer cannot legally enact a policy otherwise retroactively.

To stop payment altogether while still demand employee services would be similar to indentured servitude or slavery which has long ago been outlawed by the US Constitution.

Perhaps, what the employer is trying to achieve with these maneuvers is for you to voluntarily quit. If you do so, you'll be forfeiting several weeks of unemployment compensation for which you paid into that systems.

Therefore, I suggest that you gather all documentation, including pay stubs, work schedules, absences, etc. for presentation later at your local states bureau of unemployment compensation; their address & phone number should be in your local directory.

It seems that the employer has "constructively terminated your employment" through the use of these described outrageous, draconian methods.

The unscrupulous employer would be able to save quite a bit of money if you simply walked away & took the initial denial of benefits as fact.

Learn all you can about the unemployment compensation appeal process & document rigorously your daily, on-going attempts to seek alternative employment. Keep that resume up-to-date.

Try to interpret this "crisis" as an "opportunity" to find a better work situation. Best of luck!