I cannot find a wiring diagram for your unit so the following is general info.
Not sure what your problem is.

[COLOR="Blue"]Is there a switch that deactivates the heater after the timer times out ??[/COLOR]
Yes.
It is a centrifugal switch on the motor.
The timer switches off the motor when it slows down the heater's centrifugal switch de-activates (opens) and disconnects the heater.

[COLOR="Blue"]If so, where is it ???[/COLOR]
Usually built into the motor.

[COLOR="Blue"]I imagine that if that switch fails to deactivate the heater and the heater stays on, an overtemp occurs and the thermostat device opens.[/COLOR]
Not really as there is usually a set of contacts in the timer that also kills power to the heating coil.
Also the control thermostat or the hi-limit thermostat should regulate the heater.

[COLOR="Blue"]What other failure mode can make that thermostat open up ??[/COLOR]
Not sure what thermostat you are referring to,
If you mean Item 15 in Section 3 of the parts. It is a thermal cutoff / thermal fuse.
If it is blown see the below:

You have to find out what caused it to go.
Note: that sometimes they do just blow on their own but changing it without checking other things is a gamble. I beleive richappy (another regular poster on this forum) has done a study on thermostats and found a wide variation of actual trip point and what is specified.

Check the heating coil.
Unplug the unit and both wires to the coil.
Check it with a meter, should be around 12 ohms.
Then check from each side of the coil to the case/frame, both should be infinite ohms (open). If not the coil may have sagged or broken and is touching the case. This can cause it to run on high and the thermostats cannot regulate it.

If the above is OK then you will also have to replace the hi-limit as it should have regulated the temperature so the fuse did not blow.

You still have to find out why it blew.
Check that the belt is OK.
Check the seals (drum etc) in the unit. The air is pulled over the heating coils, through the drum and pushed out the exhaust. So any large seal leak will pull in room air and the cycling thermostat on the blower will run the unit hot.
Check that the lint filter is not coated with fabric softener residue which greatly reduces air flow.
Check/clean your vent system.
Check/clean the blower wheel.

If all OK you may want to replace the cycling thermostat as it's contacts may not be opening (welded shut).

Hope the above is useful to you.