A/C will not switch from defrost?

Yes, if the defroster is not working you may not be able to see out of the windshield. This is because the mode actuator is not working here is the location of the unit and how to change it out. Check out the images (below). Please let us know if you need anything else to get the problem fixed. It looks like the actuator is vacuum operated so you may have an HVAC vacuum leak.

A/C blows hot and cool air but won't switch off of defrost blower. Is this a safety issue?

Not a safety issue as much as an annoyance. It is actually part of the safety system for the climate control to default to the defrost mode because that can keep the glass clear and still put heat into the vehicle if the climate control system fails in some way. The mode control actuator and the cam and linkage set up on them is a common failure point once they age a bit. The plastic gets brittle and breaks. The first thing will be to check the operation of the mode system, for that you need to look up behind the dash in the drivers footwell and find the mode actuator up on the side of the HVAC case. Once you see it try switching from floor to defrost a few times. What do you see moving, if nothing moves it may be just the actuator. For that you just remove the three screws from the bracket and pull out then remove the assembly. Then remove the three screws that secure the actuator to the bracket and reverse the process to install the new one. Now you need to calibrate the sensors, for that you remove the HVAC fuse for 20 seconds, then put it back in and start the engine. Wait a full minute for the system to recalibrate and now test it out.

thank you.