I had something similar happen to my 2009 MINI a couple years ago, though mine has no sunroof.  I had water infiltration from other reasons.
My wifes 2012 car has a sunroof and had this exact problem which destroyed her footwell module. 

On sunroof cars, there is a rubber drain tube that will come off....usually water has frozen at the rubber tube, where it meets the metal drain tube when conditions are just right and the tube pops off and voila! - water drains down into your footwell module.(down by your left ankle)
  The FWM controls almost all lighting + your power windows (and maybe other stuff.)  It's expensive and even if you could find a used one it must be programmed by BMW to match your VIN.

you say yours is the ECU....are you sure?  This is a common issue (the FWM)

I literally could've raised goldfish in the cable trays on both sides of the car I had so much water.  No joke.  I had inches of water in the car.
  In addition, all my carpets were completely saturated.
I had lighting issues mostly, as salt water had wicked up into the footwell module by the wires and carpets, partially corroding connections.<<<<
You probably won't like my solution, but first I solved my water infiltration problem.  Different than yours and my info on that won't help you.  It was a unique situation.

Then what I did was cut out big (huge) squares out of the carpets and I dried them out in my greenhouse for a couple weeks.  In addition, I left the remainder of the carpet in the car, but jammed big blocks of wood everywhere to raise the remainder of the carpets off the bare metal floor so the car could  dry out.  Took months in the heat of the summer.  I also removed my footwell module a few times and took the circuit board out of it's plastic box and sprayed it with contact cleaner and scrubbed all the connectors with a fiberglass brush.

Wiggling the connectors often(for weeks) helped to finally seat them into a position where all the dozens of little wires were making a good connection.  The pins are the weak point.  Well, actually the female half of the pins (on the ends of all the cables) are a rolled piece of cheap metal instead of an expanding type of tube like they should be.....

In any case, after drying out all my carpets and reinstalling and the frigging with the footwell module I finally have my car back and it's been running good with all lights and windows for about a year and a half now.

You won't like cutting out your carpets as there is DEEEEEP foam attached under the carpets, so instead of being 1/2" thick, they're more like 3-6" thick.

It was either that or sell my car for scrap.

When I put the carpet back in I used a little double sided tape here and there and you can't tell unless you're on your hands and knees pulling at the carpet looking for the cuts I made with a very sharp blade.

246,000kms and still going.

Hoping for some advice about how to proceed. My car started leaking around the sunroof in the middle of a couple of very wet weeks here in Portland. We dont drive it very often, especially since Covid. There was standing water in the drivers side foot area. The shop says all of the electrical connections under the seat are corroded and we are looking at up to 30 hours of shop time to clean them up and *possibly* get them working. And once that's done theyll need to assess what of the electronics (ECU?) is fried.  Im trying to figure out what my options are. The engine and transmission are solid. The thought of taking her to a junk yard turns my stomach but I dont know what other options exist. Hoping to benefit from your collective wisdom.