Baking old PLA spool
Oven drying PLA

Heating filament below the transition temperature of the filament should not be unsafe when using that oven later for cooking food. You need the temperature to stay under the glass transition temperature of PLA so that the filament doesn't deform. Depending on the PLA filament, you should use the very lowest temperature your oven will be able to handle. Temperatures around 40 ℃ to 50 ℃ would be fine to leave the filament in for periods of 3 to 6 hours. Even at this temp your PLA will soften anyways at elevated temperatures, oven drying PLA not necessarily will give you good results. It is therefore of prime importance to store at least PLA (and nylon) in airtight containers with bags of desiccants. 

Stuck PLA

Moisture will not travel that far into the hotend, so changes are minimal that expanding filament has already damaged your hotend. Just heat up the nozzle and push the filament in by hand, put off the heat and pull the filament out fast. Heat up again and insert some fresh filament. Note that moisture in filament breaks the large polymer molecules, so even after drying you are left with brittle filament. Personally this has caused few of my prints to fail as the filament string broke a couple of times during prints (just snapped before the Ultimaker 3 extruder feeder); this was experienced with 2.85 mm filament on small diameter spools (about 20 cm in diameter).