Yes. You buy a SpeedFusion Cloud Data Allowance. You can chose to use it or not but deciding which devices should send their traffic through it. Changing a device from bonding to load balancing is a couple of clicks.
Ok great. 5G would be useful then. Wills response here is a good summary of current Peplink 5G routers. I agree with him, that if you want to use the speed you’ll need a latest gen device like the HD2 MBX. Recommended Peplink Model - 5g + bonded Ethernet for Remote Live On-Site Event Production (Conferences, etc.) - #2 by WillJones
The idea behind MIMO on a single modem is carrier aggregation (using different frequencies) to the same massive MIMO serving cell. cellular modems fall into categories that define how many carriers they can aggregate and what modulation they can use etc. I explained this a bit in this post comparing a CAT6 BR1 to a CAT19 iPhone. So comparing modem to modem, more MIMO the better.
However in the real world especially when you are distant from the serving cell tower, you will likely find a dual CAT12 modem router proves to be more useful than a Single CAT 18 modem device because you can use different antennas, different SIMs and connect to different operators / cell towers using different frequencies.
No. The Dual 5G option is the MAX HD2 MBX and is highly recommended. The MAX Tranist has a throughput limitation of 400Mbps so could not support another 5G modem - and remember its 4 antenna elements per 5G modem, so there wouldn’t be enough physical space on the MAX transit enclosure.