Is it possible to control a Jaguar with a potentiometer?

Yeah, but as cheap as (and common) as 100 ohm resistors are, why not?

Maybe they changed the firmware (or its a gray jag specific issue), but I somehow I remember back in 2011 (the last time I worked hands on with jags directly) a missing terminator would knock out the last jag in the string (it happened before); the last jag became the terminator. In the case of a solo jag, that meant that the jag wouldn’t work at all.

Here is a kit you can teach the kids about PWM at Jameco

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_120539_-1

The link has a funny video that is pretty good.

The grey Jags and the black Jags dont have the same hardware inside them. When TI purchased Luminary Micro (the creator of the grey Jag) they changed a lot more then software.