How about a 2nd wheelset for the mountain bike with slick tires? Wheels are easy to swap if you have quick release levers.

The cargo bike might come in handy from time to time. There are definitely times when I wish I had a Surly Big Dummy. It's a big change to transition from driving, even if only part time, to getting by with only bikes. I think 4 weeks is just not enough time to get the hang of everything. I am not sure throwing a lot of money at the problem is the best solution. I would definitely use the mountain bike from time to time. I think most of use here have at least 2 bikes for peace of mind with mechanical problems, and so on.