15 Beamer columns are basically just minipages with some `\hfill` between them to distribute them across the page (and a couple of extra features like making them overlay aware etc.). So I would not say "don't use minipages with beamer" but rather that is normally easier to use columns instead of minipages.
10 It is straightforward to store the information in an aux file, and this had already been done in [this post](https://tex.stackexchange.com/a/539395). You can then read them out with a pgf function, which I called `MMapNodesAtLevel`. (With these functions one has to be a bit careful, when one tries to define a new function under a name of an existing function, one gets an error. Hence the long name of the function, which makes it less likely that this function is already used by another package or code.) This function has the number of nodes under a level after the first compilation, when this information is retrieved and stored in the aux file. You will want to give the mind maps names with `mmap/name` in order to have unique numbers per mind map. This makes it possible to have several mind maps in the document for each of which the information gets stored in the aux file. If the information is not known, e.g. because this is the first run or you invoke the function outside the scope in which the name of the mind map is set, the function will return `0`. This is why we use
10 The situation is a bit tricky because even if it won't take up any space, the `tikzpicture` will still have to start a line and `tcolorbox` you use afterwards will go in a fresh line.