The middle knob is much like the toggle switch on the other pedals: it’s populated with some well thought-out starting points which you can then tweak and save to one of the preset slots. However, last time I tried, those 7 bases are fully replaceable too so you can design your own starting points too.
Time-based effects are only one at a time, so delay or chorus or reverb. The other stuff is mostly available together. There is something to do with the glitch section that has a minor limitation, but I forget what that is exactly.
And yes, all controls are fully assignable to be whatever you want. There are two soft control sections, each of which allows a knob or footswitch (and hopefully in an update the bandwidth toggle switch too) to control up to three different parameters with customisable ranges (both in parameter values and to which portion of the knob travel it is assigned). It’s so powerful. Definitely wish the C4 had some of this ability. Hopefully an updated/revised version will bring some of this innovation in.