As you said you can´t solder aluminum, you would have to weld it together. If you can easily get yourself a welding tool for such purposes you can go for that method, otherwise it´s quite expensive to get such thing. If your lucky it´s only an aluminum surface with copper under it. If so you can scratch the surface a little and try the soldering method.

The batteries are "stupid", they do nothing more but taking charge. The board you solder to them is the "brain" of your powerbank. The board decides if the batteries should get charged or not, so it depends on the board you use.

To your other question:

When you open the powerbank and grab another case it´s possible to increase the capacity of your powerbank. It´s similar to the guide, but you have to test how the batteries that are already in your powerbank are connected bevor you add your iPhone batteries.

Yea, that´s because they just put in their components very cheap (different cases were the soldering was really bad). They do only the minimum they have to do and in the end you buy some cheap tuning kits for a lot of money. Techdemy, an YTber from Australia covers that in different Videos when he repairs Scuf controllers.