At any time during your recent adventures (e.g. when the rear linings delaminated) did the brake pedal ever travel significantly further than in normal use?
The first thing that comes to mind from your description is a master cylinder seal issue. The biggest pointer there is the pedal falling while your foot rests on it. Strong panic braking is also consistent of that because it's a sudden spike in pressure and a slightly damaged seal will still hold. Slowly falling pedal is a peak somewhere - oozing from the piston, leaking from a hose, or leaking past the master cylinder seal. While technically it could be an air or fluid "leak" past whatever seal it is, air isn't going to suddenly compress a little easier while you're holding your foot on the pedal, and a big bubble or air would give you panic stop issues too.
Something I don't feel I see mentioned enough - as cars age the master cylinder bore gets polished in the range where it is most heavily used. This makes it smooth and slightly larger diameter. Past that point the diameter goes back to original and there could be surface imperfections. Grind the seal past that, like during bleeding or some weird brake failure that causes the pedal to fall dramatically, you can now get a little tear in the master cylinder piston seal, for which there is no fix but replacement.
I see lots of bleeding instructions, including BMW's own, which say "pump the pedal to the floor". I specifically place a brick or block of wood under the pedal to keep it in normal operating range on my vehicles.