Hello cloudge,
In the SDK of FRDM KW41Z you can find the following projects: end device(end_device), sleepy end device (low_power_end_device), REED (router_eligible_device). For a device to become leader or commissioner, according to Thread specification, there would be a "competition" to see which one of the devices from the mesh would become the leader or the commissioner.
Also, REEDs become routers as the mesh needs them. For example, you won't have 6 routers in a mesh with 6 devices, as they are too many, like you won't have 1 router in a mesh with 100 devices, as they are too few. The protocol knows how to assign efficiently these roles, so they would be spread proportionally. You should not be concerned about this, the protocol was specifically designed to take care of this. Also, if somehow the only router breaks, the mesh observes that and will automatically promote a REED to router.
The single role that a KW41Z cannot handle by itself is the border router. The reason is that the chip doesn't know WiFi so it couldn't get out in the Internet. The only protocols known by KW41Z are: ZigBee, Thread and BLE(Bluetooth Low Energy). If you don't mind not having access to the mesh from the Internet, you won't need a border router. As the boards know BLE, you can design for example an android app that controls the mesh network. If you want to have access to the Internet, you need the border router, and the best and cheapest choice is the FRDM K64F + MCR20A sandwich. K64F knows how to communicate via Ethernet and MCR20A knows Thread protocol. Take a look bellow how it looks like this border router.

You can modify the applications from the SDK so they will behave as you intend.
You don't need the IAR, all the projects are also available in MCUXpresso, a free IDE provided by NXP with compiler included. If you worked with Eclipse, they are very similar, it is Eclipse with an NXP plugin over it. It is available here for download.
Best regards,
Lilian