If I am my mom's POA for healthcare, and she gives me permission, can I check her test results and send messages, etc., on MyChart?

It’s fine. I have an adult son with complex medical conditions and a brain injury. My email is used to access MyChart and whatever other patient portal systems his care teams use. I do all the scheduling, checking results, requesting refills, and messaging medical staff with no issues at all

Yes, all you need to access the info is your mother's permission. Make sure you have all her passwords; don't just have her sign in and then you take over the computer. (In case she has a medical emergency or becomes incapacitated.)

If you send a message to a doctor or provider, they could ask you for the POA, so have it handy, but they also might not, especially if they have met you when you've taken your mother to appointments.

Yes. You have every right to access her health care information via any route with her permission, no MPOA needed, that is just an extra benefit in the event she becomes incapacitated.

I would encourage you to have a HIPAA that never expires done by an attorney, that will allow you to do what you need to for the rest of her life.

I have no idea what MyChart is.
If this is an online link for your mother's lab/hospital/testing, she should provide you with the sign ins that she uses. I use Kaiser. With my Kaiser password my daughter or my partner would be able to function to look at all charting and lab testing and etc on MyKP link.

If you have any questions, speak with your mother's medical team or the Hospital or Insurance program in charge of MyChart. They may want you to present them with you POA papers.