The side walls are actually very important for the structural integrity of shipping containers. If you cut holes in it, it absolutely needs to be reinforced or your new roof line is going to sag over time. They're built as cheaply as possible to accomplish their exact task, which is holding a bunch of crap and being stackable, and the side walls provide both tension to hold together the frame (basically a box) and to provide rigidity to both floor and the top. I reeeally hope y'all reinforced the areas around windows.

You can do a bunch of things to shipping containers to make them viable, but it's far more expensive in materials and time than just building with more raw materials. You can remove the toxic coating but you have to have a whole human do the labor for that and do is safely, otherwise that human is paying for that decision with their health. The materials and labor for that aren't nothing.

When you look at what is actually used from the container in order to create a house, it's really not that much. Basically just framing of questionable stability and some corrugated metal siding, possibly the cheapest and easiest part of building a house. I can personally frame up something container-sized in less than a day, easy peasy. Siding and drywall are also easy. The harder parts are everything the container doesn't provide: foundation, vapor barriers, plumbing, electrical, a good roof, any specialized need for insulation, efficient ventilation / heating / cooling, making sure safety elements are up to code.

In terms of mobility, I would not recommend moving a container home that has been substantially modified unless it's been upgraded for exactly that. They can only be safely moved using the corners to distribute the weight, hence the special container arms at shipping yards that grab the corners. If you put a custom roof on there or otherwise make it so you can't grab those corners, there's a good chance the whole thing falls apart unless you've reinforced it to be mobile using yet more investment.