How hard is it to become a warrant officer in the reserves and as lower enlisted?

It depends on the Warrant field you're trying to get in for. For example, the Food Safety Warrant requires at least 5 years as a 68S. It would literally depend on which field you're trying for. Check around your Reserve Center Retention office. There will probably be some pamphlet literature about Warrants with email/phone contacts.

It depends on the specific MOS, but, in general, I believe that pilots are the only MOS that does not have a rank requirement. Consult the requirements on the Warrant Officer recruiting webpage.
https://recruiting.army.mil/ISO/AWOR/ARMY_FEEDER/

Very hard...virtually impossible.....however, not saying impossible....speak to a WO recruiter or go to https://recruiting.army.mil/ISO/AWOR/ARMY_FEEDER/