When my medium speed went out, it was the switch. Most of the time when you only loose on or two speeds, except for high, it's the resistor pack. But as I said sometimes it can be the switch.
Yes, the blower motor can cause your problems. But that is only because of a poor circuit design. As you have found out yourself. The blower motor begins to wear with time and starts to pull more current and because of the design of the circuit the weakest link goes first. In our case the resistor pack and speed switch fail. Some to the point where parts melt. Does this mean your blower motor is bad? No, it means the circuit needs to be redesigned. The first five years of my JEEPs life, I replaced the speed switch, mode switch and resistor pack at least 3 times each. Then I discovered if you were to install a relay between the switch and the resistor pack/blower motor it would fix the circuit design. JEEP seemed to think it would be okay to have a low rated amp switch control a high amp blower motor.
As we know this does not work very long. So the fix has the switch controlling the relay which in turn controls the blower motor and all is good. This works because the relay acts like a buffer in circuit and protects the fan switch, mode switch and resistor pack from failing. 9 years ago I added the relays and up until this past fall on the original blower motor where it just quit spinning all together.
Many have done this upgrade and if you can solder takes maybe 30 minutes and less than $50 to do. It was one of the best upgrades I've done to my JEEP, well along with the headlight wire harness upgrade.