All correct re Verizon, see http://forum.mikrotik.com/t/miktroik-and-verizon/164117/1 You don’t have to swap IMEI on modem, but you do need a IMEI from CERTIFIED device to activate (that isn’t already active). Even an AT&T phone may work for Verizon to get a SIM car, since some manufactures don’t know what carrier may use there phones/hotspots they submit them to Verizon anyway.
AT&T will work fine with Mikrotik device, technically they require ODIS+LwM2M support, just not very draconian yet when activating a device & Mikrotik IMEI have generally worked for AT&T. But be aware that Quectel in the US models don’t support all of the carrier aggregation modes available.
Personally, I use the Telit LM960s in Mikrotik’s -R models instead – doesn’t help with Verizon (although the Telit 960s are certified by Verizon, just the device isn’t) – but the Telit’s supports all of AT&T’s CA modes and Mikrotik provides all the RSRP/etc and CA mode stats on the Telit modem, so they work pretty well. Only downside is they are expensive.