I would say volunteer as an 'employee' implies you are receiving compensation.
Assuming the patients you are searching for are not in your (hospital's) system, then yes, you will need to gain access to the EMR.
I would think to be considered an employee, you'd need to be compensated. I worked on a research project at another institution, was not compensated and was probably considered a volunteer, but also got a temporary email address and access to patient records for the project.
You’ll have to be affiliated to the institution somehow in order to get legal access to private health information. This would likely entail applying, doing some learning modules, etc. Maybe not if all the data are privatized without being traceable.
Unless you’re getting paid, I wouldn’t consider yourself an employee. Not sure what you would be, but not an employee.
Neither, probably. You're a research associate. But you need to do something to be given access to the EMR, whether it be passing research requirements at your home institution or the other institution.
You would be considered a research assistant, but that doesn't mean employee, for med students it is usually a volunteer position. You will have to go through paperwork to get all the necessary authorizations to access the EMR.