Thank you for thinking along, all your points are valid and have been considered, there are fairly obvious drawbacks to doing it this way but simpler options are - alas - not on the table.

This is the bike (image ripped from an ad, not that bike but the exact same model):

http://imgs.advertentiex.nl/upload/Te-koop-prachtige-christi...

So you can see that in the back there are no options other than to either completely change the frame or to go for a single speed and neither is feasible. I could maybe expand the space between the forks in the rear so a wider hub with a motor + cassette would fit but that risks cracking the aluminum frame.

The bike has a rear hub that is geared, so there is no space there and this being a cargo bike means that an after market mid-motor can't be placed (it would run right through the middle of the main spar. That leaves the front wheels for the electric drivetrain. I'm not afraid of slipping or sticking, electric motors tend to be pretty well behaved in situations like this, the difference in speed between inner and outer wheel in a turn will be readily absorbed by the motors and discarded as heat, and they won't be overpowered to the point that they will start turning independently, each motor will run at a leisurely 125 Watts or so. The one situation where that might happen is if one wheel comes off the ground but in normal use that should never happen.

This is the kind of kit that I'm eyeing to do this conversion with:

https://www.fietsunie.nl/Babboe-Big-bakfiets-elektrisch-make...

This is a pretty common thing here in NL and rather than build the whole drive train from parts I'd go for a kit.

If it comes down to cutting/modding the frame, I guess it is time to start looking at other options. The one thing I've noticed is that the gearing seems unnecessary while using the motor. With pedal assist, starting from a dead stop is quite simple. I'm sure that there necessity will be obvious if I ever find myself without power in the battery, so I'm guessing removing the gearing is a non-starter

One of the people using this bike has a knee injury so gearing is an absolute must and that is the same reason why we are looking to convert the bike to electric drive in the first place.

I have used a bike with single gears and I currently run my own ebike with a single gear - it's really not that bad if you are in shape. I have pretty strong legs though so ymmv.

Yes, that's exactly the point for me just by myself it wouldn't be an issue.