Thanks as always. I was under the impression that MMM was needed to install Wads and Forwarders. And ios (which I admittedly should be up to date with after using your syschecker in ModMii). Have I got this wrong too?
To install titles from a wad, you need a wad manager - MMM is just one of the many ones (the most modern ones are Some Yawmm Mod and Wii Mod Lite, but the latter is somewhat unstable)
Similiarly a wad is a wad, regardless of what it contains (an application, DLC, IOS, ...) - in particular, "forwarder" is just a name for an application whose main purpose it to load and run another one, but has absolutely no technical difference from a "regular" channel
Regarding the bootmii on sd card, do I then only need this sd card for creating a NAND and potentially restoring if it bricks at some point, i.e. Don't need to keep the card in but save it for if the wii bricks at some point?
Bootmii is made of two parts - the one installed to the console (as boot2 or IOS254) just mounts the SD card and runs /bootmii/armboot.bin, nothing more
The official Bootmii interactive user interface is composed of two files (/bootmii/{armboot.bin | ppcboot.elf}) and both are required for it to work, but there are other uses of Bootmii where you won't need a ppcboot.elf (such as for Wii Linux, iirc), and that's possible because the on-console side doesn't care for loading ppcboot.elf, that's the job of the armboot.bin (which may of course do something different than loading /bootmii/ppcboot.elf)
The nand backup, which can be created with multiple methods (of which Bootmii is the better), is typically stored as nand.bin and keys.bin on the root of the SD (actually, keys.bin is included in the nand.bin, but it's stored separately 1- for convenience since some tools accept it 2- since as long as you have your nand key you can convert someone else's nand backup for use on your console), you don't have to keep them on your SD though of course more copies can't hurt
To restore a backup, if you ever need to, you could use:
1- Bootmii booted from boot2
2- Bootmii booted from IOS254, in which case not only you need a gamecube controller due to an artificial limitation built into the official GUI, but also a way to run IOS254, which typically is Priiloader meaning you have access to better ways to fix the issue you're probably trying to fix)
3- A hardmod (external programmer connected to a PC)