Freezer temp unit maintains 18 degrees on the lowest setting

no rule of thumb
maybe if you got a different thermostat that has a lower setting than standard
however as upright freezers are very bad freezers compared to chest type freezers the lowest reading will be around the -35 degrees C
Why very bad -- every time you open the door you dump 20 degrees out onto the floor and the temp difference is more pronounced than opening the lid of a chest unit which keeps all of the cold in the unit
remember the colder the setting the more the compressor is working

The manual for this unit can be found here.

Control board is miscommunicating with the rest of your unit. Contact a professional.

You should be able to set the unit up to 30?F and still freeze some contents.

it may be possible. Turn the temperature on the lowest setting and see what happens. If it still freezes everything up then it is not posible. If it doesn't freeze everything , then check the temperature with any temp probe. It should run at somewhere between 36 and 38 degrees. This is the normal cool setting on a refrigerator.

Over what period of time did this variation occur?

The freezer should have an automatic defrost cycle. The temperature will rise while it is going through this cycle.

The light should reset itself when the freezer gets to less then 18 degrees. If it is still above 18 degrees it will stay on. Now if it is cooler then 18 degrees then unplug the unit for 1 minute and plug it back in once you are sure that it is below 18 degrees. But do not rule out the thermostat. It could have blown out when the brownout happened. To test it jump the wires together and see if the freezer comes on and goes off

not freezing icedraem is an issue, bread not so much, different breads contain different ingrediants that may or may not freeze, forst offthough wher is unit located??? in garage??? if so be aware that it may not operate efficiently ion ambient temps lower then 55 degrees, but if this is in your kitchen i would suspect a "refrigeration"/sealed system issue, perhaps "low on charge and not able to maintain a frozen set temp. Meat may stay frozen ( for a little while anyway, but the ice cream is the first to show indications of problem

not necessarily, is it maintaining temp? all food staying frozen? especially ice creams if stored in there. most newer freezers displace the heat generated while cooling the food through copper lines running through sides of unit, helps to cut down on air temp differences between the inside (0 degrees) and outside temp to prevent moisture and possible condensation causing premature rusting of unit

you could try a 3 in one hard start kit... comes with a copacitor to help start compressors with age and having trouble starting