I think number 1 is especially important. It seems like it would probably be part of the baseband firmware to address an incompatibility between the Motorola LTE chipset and Alcatel-Lucent LTE base station equipment. It's probably the reason some people have issues and others don't. My guess would be that people without issues on firmware prior to .901 are probably connecting to towers running different base station LTE equipment.
Not sure about IMSI, I thought that was a unique identifier attached to the SIM card. Not sure what dual IMSI support would do on a device with only 1 SIM slot. Maybe the LTE SIM can hold 2 IMSI #s, 1 for 3G and 1 for 4G? If so, maybe they are going to provision dual IMSI numbers for people with data issues that aren't solved by the update.
Number 3 might be pertinent to 3G mode, as well; I think what it means is that it will properly fall back to 3G if it cannot connect to the 4G network.