The MX uses wan links in following way:
WAN1 -> WAN2 -> USB -> SIM slot LTE
SIM slot is integrated in MXs with C for cellular.
Load balancing is for WAN1 and WAN2 so as long as you use MG21 coupled to your MXs WAN1/2 ports you´ll manage to do it or as you mentioned use a 3rd party supply.
LTE is considered as a backup solution on Meraki so you can run MXs on a LTE link and its shows up orange in dashboard. I would like to see it enabled on all of the fancy sdwan thingies we´ve got.
What is your use case for LTE as primary wired as backup?