Do I need to have medical POA or be a conservator in order to hire someone to bathe my mom? She's combative and refuses to shower.

Perhaps you could try the no rinse products; they're available as soap and shampoo.   She can remain almost completely clothed as the products are applied, one small area or larger, at a time.   They're somewhat like lotion.  

I used them during a hospitalization and now sometimes at home.  They were also used in lieu of immersion bathing at facilities where my father received rehab.

They're available at drug stores, but shop around as they can be expensive.

I'm not sure if ANY 'power' you might hold over mom can make her shower. You can hire outside care as much as you'd like, but they'd have less 'control' over getting mom to bathe.

Something as personal as bathing--that's fraught with drama from the get go--if your mom doesn't want to bathe, not much you can do. My mom lies and says she showers every day but when you check the shower or the towel hanging in the shower, you shouldn't find COBWEBS....

You can check with agencies that work with the elderly and specifically ones that are trained to help with showering/bathing. BUT, the client HAS to be willing to meet them halfway--meaning, they have to WANT to bathe. It certainly is worth a try. Maybe mom would be more amenable to a 'personal assistant' than family. You never know.

Doesn't really matter if you have POA or not--this is an iffy situation. If mom is of sound enough mind to know how to say 'no' to something she doesn't want, she has that right.